I think this is the most important point. You essentially never went out of bounds. If the resort wants to declare an area out of bounds, they have to actually rope it off. A lawyer could have a field day with this. Me, I would have just said to the goons "then you should rope it off up the hill".
I dunno, about this, it would depend on the geography a bit. Consider the case where the a single run branches into two that run alongside each other, with trees between and they rope off one the the entrance to one of the branches at the point of the split. If you go a little ways down the open run that you're allowed to be on, and then cut through the trees you're pretty obviously getting onto a run they don't want you on.
If it was my first time going through the trees and I found myself on in this situation I'd expect ski patrol to cut me some slack. If me and my buddies were essentially doing laps where we were cutting through the trees to ride a roped off run I'd have trouble justifying this and would expect ski patrol to call us on it.