Sorry about the late reply; some days (weeks!) I don't get much of a chance to check the boards.
I so understand the concern about trunk and head control. One of my guys is a spastic quad, in a wheelchair. He has some issues with trunk control, and milder issues with head control. Just so you know, he doesn't need additional bolsters for his trunk on his chair, and doesn't use a head support. However, independently, he has some troubles.
Initially, I had told the kids that they were probably going to be too short to ride MS; but now they're 7 (and smarter than Dad), so they found out on their own that they were tall enough. So, I went up and talked to a few CM's, and it sounded like it would be ok. Unlike RnRC, where there would be a chance he could fall out, I knew that this ride would be safer. But I was still concerned.
We already talked about the boarding; the CM's were great. DS can help us transfer him, taking a few steps. Someone who was totally dependent on transfers might have more difficulty; the chair is right out side the "pod", but you have to make it a few steps.
The ride itself does "push" you back into the seat, giving you more stability than you might think. Also, there is a fairly robust "harness" system, that comes over your head. DS can totally come out of his chair if not buckled in, and he always comes off the couch, etc. But in MS, I was very confident that he would not slide down.
I was also concerned about his head control. I took the chance a couple of times to take a peak at him during the ride (and almost tore my head off doing it), and he seemed fine (and he loved it!!!). I think the G-forces also helped stabilize his head, as well.
I hope this helped! Sorry about the length, but this was one ride that I spent a lot of time debating on letting him ride or not. Now, he can't wait to get back on it.
Wade