Mum has late stage dementia and lately her tongue is protruding all the time. Has anyone else experienced this?

I was suggesting that for comfort reasons. It comes in a spray bottle.

I would check with a speech therapist. In the meantime, perhaps you could try some biotene. Many medication make your mouth dry.

In late stage dementia they lose much of their muscle control/tone. You may see them get more rigid and difficult to move. Your tongue is a muscle so can be effected. You will probably see them pocketing food and forgetting how to swallow. As said before, trying to feed at this point could cause them to aspirate.

This can sometimes be a serious side effect of medications. Check her medications with her Dr. to see if this may be a side effect.

Trying to feed someone who cannot properly retract the tongue and swallow is a real risk for aspiration pneumonia. Think about asking Hospice to do an evaluation on mum.