My son at some point a few years ago decided to stop eating meat on his own, struck us as odd but we just moved on to other foods. I think the allure of sliding around on surfaces is a guy thing...I find myself thinking that's what I would be doing if I could when my son does it.

Our son is also on the gf-cf diet, and is doing much better being on it. He has been gluten-free for 2 years now, and casein-free for 1 month. When he goes off the diet, we notice immediate gas and more silliness. Also more grumpyness - he's very quick to get angry. When he follows the diet, his mood is more steady, and he is far less oppositional.
I had let him get whatever he wanted when his class would go on a field trip and eat at a restaurant...but this year, I've decided to see how much better he can feel (and document the results) when he takes his own (gf-cf) food on those trips. He has been disappointed with me in doing this, but I can weigh things in my favor with special treats (gluten-free cookies, Italian ices, and pizza). But you're so right - the worst scenario is to walk into a situation where you're surrounded by tons of goodies that your child should not have!!!
There are many kids at our schools who have allergies to nuts and dairy...and the schools are very accomodating in modifying the food offered so that these students will be safe. I believe that they need to do the same for our kids, because this diet for autism is a medical choice, and one of the therapies most often recommended for autism.

Oh, I so understand that desire to just give in and see if it'll work. Unfortunately for us, it always backfires. Food related events are always stressful. I've actually started bringing "ok" goodies to everything! Ugh!