My wife earned her first degree in Music Therapy and worked in the field until we had kids and she stayed home.
It's hard to point you in any good direction - but know that music therapy is more therapy than music, and piano is an unlikely candidate.
The basic premise is to avoid the YAVIS principle. That is, that Young Affluent Verbal Intelligent Successful patients do very well with talk therapy, and the other 99% of them are hard to reach. So you branch out into music, dance, sports, occupational, etc., in an effort to connect.
Music therapist are not required to play at high skill levels. They deal hands on with very low functioning patients - in my wife's case with profoundly retarded, adults who functioned at a 3 year old level. People who are not continent and will never learn to dress themselves.
Piano isn't good because it interferes with you getting your hands on your client. Almost any other instrument isolates you less. Of course piano is a required course in the curriculum for other reasons but it is of little use in actual practise most of the time.
Tim, I would not argue with a word of what you wrote above, because quite obviously this is a field you know a great deal about. And I don't.
I do want to add that at this time I am working with two students who are described as "high-functioning autistic", and I'm going strictly by the seat of my pants. I'm not a lable person, and I'm not sure the above label is even useful, much less accurate. I do know that both these students have rigid rules, feel comfortable working in a more limited framework (can't describe this very well) but are absolutely wonderful to work with.
Perhaps other instruments would help even more, but both seem to make a connection with the piano and love the structure of the instrument.
This makes me wonder if different people are "helped" by different instruments, depending on whatever strengths and limitations they have that might get them "letter labels".
I also don't know where the positive effects of playing any instrument stop being or start being therapeutic. I bow to your knowledge here.