In one of my pages I have a div that is sumitted back via Java and Ajax. Prior to submitting, I validate the individual fields using .validate().element( "#myelement" ); I do it all in javascript though, and don't rely at all on the built in automagic asp controls.
I hate to say it, but that global form issue is your real problem. It's probably not an option, but if it is, look as switching to asp.net MVC.
