What a wonderful challenge, Penguin! Lots of work with planning, though. I would love to do that myself.
I guess you have the up side that their expectations would be realistic. The girl and mother woudl be more familiar with her struggles and limitations than you would ever be, so they would appreciate every thing you gave and every bit of progress she made.
I haven't much to say, because I don't know a lot about this type of disability. I would ask what sorts of therapy they use for small movement control, andadapt these to playing the piano. I think you would probably need a lot of larger muscle support so that she could concentrate on the smaller muscles - so you would need a well suited arm chair at the piano.
Reading music wouldn't be involved, so I would build a good theory base. It sounds like she has a good ear, and possibly a good musical imagination. I would develop a more wholistic music education, with an awareness of styles. Get her to compose from day 1, by singing the notes and you wriitng them or her giving you letter names. Use the singing as a central support of her playing - so have her sing everything she is trying to play and have the playing as something that supports her singing. Play lots of duets.
It sounds like so much fun, if you can manage it with time and everything.
I'd love to hear advice as well, and also to hear how things are going form time to time if you do take her on, Penguin.
I have been approached recently about taking a 4 year old deaf girl. That is not definite yet, but I am hoping it will be. I know it's an entirely different type of disability, so I don't know if I should be mentioning it now, but I am really interested in exploring different approaches to teaching special needs students.