The dynovox or devices like them do work for some well , but as in your mother case when movement becomes a problem then the communication boards are needed. I have searched for sometime on the different comunication device and boards. I haven't used the EZ Picture Board , but If my husband was still living I think it would work the best for what we would of needed in a communication board.
It seems the EZ Picture Board would make easy communication using the board and would work well for what you would need it for. It has a pain chart so you can know if your mother has any pain and the other side has a list of " I am" with pictures she can point to & "I want " to point to along with A-Z so she can spell out something and has numbers. It also comes in different languages at the same site http://www.bindependent.com/cgi-bin/shopper.cgi?preadd=action&key=snc100 .
You could also make one that would be something on the same order. You can make a board by cutting out of pictures from magazines etc and glue those to a card or poster board, just make sure you use permanent marker so it wont run.