Am I able to receive nutrition counseling from a dietitian on behalf of my mother, or would there be any restrictions based on HIPAA?

You may be able to arrange a nutritionist consult simply by asking your MD who best to see. You would not be giving personal information about your mother other than to say for instance "Female 79 insulin dependent type I diabetes for ___ years. False teeth and difficulty chewing and blah and blah and blah." You are simply making a profile and not speaking of anyone specific if you see what I am saying. You are asking for nutrition consult.
If you cook for your Mother then her doctor should be able to order nutrition consult online or in hospital that you can attend with your Mother's permission. That way she can give preferences and etc.