I have mixed feelings about this. I have been on both ends of similar situations now, as in being the classroom teacher, the sped teacher and being one of the "outside" team. However, the students I work with now don't have that level of physical issues, rather severe mental health issues. Most likely, mom has been treated horribly somewhere else. And Veronica is correct, cooperation will work a lot better!
I think your principal or sped administrator should make it clear that classroom teachers are not the ones who do the modifications, and be emphatic about it. I mean, I did them on the fly when I taught English, but it seemed "normal" to me, coming from a special ed background, and it's also a hell of a lot easier to do for a writing or reading project than science.
However, I have to say, the world outside of education will not understand when you are upset about "having to stay beyond contracted hours." They see you as professionals who stay until the job is done. Again, your administrator needs to be the one to set the parameters about this. And, I would demand (nicely) some type of report/eval that would specifically tell you what exactly is going on with this kid.
Many years ago, when I taught in AZ, I had to spend a couple of prep periods "subbing" for the teacher of severely mentally/physically handicapped students when she went to a meeting (this was something we all had to do). The students were so impaired I was nauseated by some of the physical aspects. Then, I felt guilty for feeling this way. These kids would not be in a public school setting in Massachusetts, but we served everyone in public school programs, which is much more cost effective than having towns pay for kids to go to private schools. I remember some parents who were encouaged to give their kids up to the state, so they could get residential schooling or go to a therapeutic school. That doesn't happen here, but all of these severe cases are really tough to deal with.
I know that when I go to meetings at the school of the one younger client I have (the one who almost destroyed my office in a tantrum last week), I make it clear that I am "one of them," and aware of the constraints the school has, but also I make sure they understand what issues the kid has. Of course, I am not there as an advocate. Those people are often brutal.