Unfamiliar with the advanced state of these diseases at which I guess any technological solution is largely ineffective, but in the earlier stages would it be useful perhaps to add some simple physical interface options, like a big button you can mount to the wall or whatever, or a novel audio command interface? Like four loud noises in sequence means call someone. BANG BANG BANG BANG. Should be feasible to achieve if any muscle control remains.

In terms of custom hardware, a MEMS microphone + small microprocessor on a small PCB with any form of interface to the computer (eg. USB=power also, bluetooth/wifi=needs battery) should do it. If presenting a keyboard interface over USB, should be OS independent. Other solutions may require a small client app.