Has anyone found a “reacher” type tool that allows an individual to pull up their own adult diaper while seated on the toilet?

You might try one of the "grabbers". If you haven't seen them, they're about 2 +/- feet long, with a handle that can be squeezed to close the grabbing mechanism at the end.

Thank you; the lengths available are too long for my Mom’s very round (apple type) figure. Neuropathy and inability to reach back with her dominant right hand are additional complications.

Does mom have a grab bar on the wall next to toilet?

I would check a med supply site such as active forever
I know there are helpers for socks which might be too long

I'm wondering if the problem is just because the disposables fit so snugly, or does she have trouble with normal underwear and pants too? There are some pretty big absorbent overnight incontinence pads that go in normal underwear, if she is still using the toilet but wears the disposables just in case then the pads may be all she needs.
Another thought is to try suspenders/braces clipped on to the underwear to help pull them up... not a very trendy fashion statement though.😕
I don't suppose she would consider having someone help???

Thanks to those answering. Mom’s problem is 1.She will not ask for help. 2.She has a very round protruding stomach, so trying to pull the backside of her adult diaper is impossible for her to accomplish because it is underneath both her bottom and her stomach. Imagine having an inner tube around your waist, even after rising from the toilet, getting the back half of the diaper up and around her body is very difficult. I have suggested using OXO brand chip clip holders on each side, as far back as she could place them to give her a firmer grip on the flimsy fabric of the diapers. She most likely has tried this with dollar store chip clip holders which will be plastic with no grip & likely smaller than the ones I tried. Today I cut a pool noodle into a hand length piece (6 inch-ish) and then cut it lengthwise to create a slit so that I could slide the top edge of a Depends into the opening. I have a much stronger grip than Mom does, so it kind of worked, but was still too slippery to be practical. I have been researching grip tape to apply. I should have paid more attention to the Science projects that my sons as 5th graders were encouraged to invent. Perhaps a small hoopla hoop with grip tape around it, so that Mom could wrap the top of a Depends around the whole circle then pull it up would work.Thinking outside the box trying to solve this issue.

Maybe apply a handle of duct tape to the depends before trying to pull them up. I'm visualizing putting the end of the strip of duct tape to the outside of the depends then folding it back on itself so the other end tapes to the inside. Do it on the right and left side. If this works you could pretape all the Depends all at once so they are all set to go.

Honestly, anything that you can cobble together may work for a while, but unless her need is very temporary I imagine the pain-in-the-a** factor will soon kick in and she will abandon it. (Kinda like all those "useful" kitchen gadgets we all have hiding in the back of the cupboard).

I'm still wondering how she manages to dress herself otherwise, does she have help? Does she live in her own? I think the most practical solution is to have someone help her, one of the inevitable indignities of old age.

She doesn't have to pull them all the way down below her knees. She can lower them just enough to sit on the toilet and have them out of the way of her privates. It might take a few tries to get it right. If she keeps her legs apart enough to keep them from falling to the floor when she gets up, that should do it.

I purchased Tena briefs for my husband using Amazon to order them since I didn't find them for sale in my local pharmacy.
https://www.amazon.com/TENA-Flex-Maxi-Briefs-Large/dp/B0013Y76AY/ref=sr_1_1_sspa?ie=UTF8&qid=1508855492&sr=8-1-spons&keywords=tena+flex+briefs&psc=1#customerReviews
I see they are sold at Walgreens, though.
I gave them a rave review. You might be able to use their design with another diaper--I'm not sure. Tena is expensive: about $30 for 22 briefs vs. $37 for 48 briefs for Depends. The advantages of Tena are that it is smooth and made of a material that is breathable so the likelihood of bed sores is diminished. It is very very absorbent so it might last overnight. The best thing is its design. It has a wide belt around the waist that attaches with a velcro like stick which is reusable. Then the absorbent pad hangs down from the back and is pulled through between the legs. This pad has velcro attachments on the left and right which then stick it to the waist. Before going to the bathroom, the pad gets pulled off the waist. Then it can be pulled up to the side and reattached so it is out of the way. After going to the bathroom, the pad can be pulled through the legs again.
https://www.youtube.com/watch?v=0yZQlaEk1ww This youtube video shows how the product is used at about 20 minutes into the video. There are 2 ways this product might help your mother. First, if she is able to accept some care, you can pull the pad down and reattach it to the side before she goes into the bathroom which might allow her more dignity. Then you can help her reattach it after she comes back. Second, since the pad is light it might be easier to handle with a grab it tool.

Diana here is what I did, I purchased 2 pairs of 9 1/2" sponge forceps from Amazon.
The ends lock so there is no problem with them slipping off the underwear. They are long enough to use to pull the pants up over the feet. I had explored the long handled cleaners and decided I did not want to handle the dirty TP. having worked in the OR I knew about the sponge holders. They are or probably were used to wrap a gauze swab round and dip in antiseptic to clean skin pre-op. These days the OR staff seem to us disposable gizmos as skin cleaners. Not expensive, they cost about $8- 9 each. some of the other healthcare sites probably carry them, i just have not looked

To be honest with you, if she can't bend over due to being heavy, then it's time for a diet. Sadly, so many people are too heavy and some to the point they can't even bend over and touch their toes. Obesity is on the rise and I'm glad part of the solution is starting in schools taking action to combat obesity since obesity is the cause of many health conditions. It's never too late to follow the example of some schools taking steps to combat obesity

Did you realize that much of obesity is caused by meats with steroids in them? Ranchers and feed lots feed cattle, hogs, and chickens steroids to "bulk them up", what do you think they do to the people that eat them?

Backscratcher with curved scraping area worked for my mom of similar proportions.

Maybe instead of "pull-ups" you could try the old style "boxy" type with tabs. Think diaper here but don't use that word with Mom.

Medicaid/Medicare used to only give those for free and I know they're cheaper.

My SIL has that problem but her DH helps her. I only know because we ate out one day and I stayed in the bathroom in case she needed help - I wasn't aware she would need that kind of help but I jumped in, disposed of the old and helped her to pull up the new. That's when she said Howard usually helped her but he didn't follow her into the ladies room that day :)

I too thought of suspenders. Perhaps you can keep a walker in the bathroom to be used like support bars. Hold on with one hand and pull up with the other?

As for reaching behind - I was surprised but you can buy a home bidet (spelling?) that fits on your toilet and will clean you back there! Amazing. I haven't bought one but the reviews at Amazon.com are amazingly positive.

There comes a time when she must ask for help.