A weight restriction for coasters definitely sounds like just a rumor. Now on some types of flats it might make more sense (I know the parachutes has a weight limit), but even so would they ever actually have scales to enforce it?
I've also had the same arguement that Matt had with someone I met on before college classmate get together thing. I insisted if she had to sign anything at all it would be a photo release most likely, and told her she should really read stuff before signing it, haha.