waq,
one rolling session won't magically fix anything. In fact, I'd say that squatting itself is more likely to help you hit depth than foam rolling, which has other uses (e.g. not getting tons of adhesions, helping recovery etc).
Can't remember the exact passage, but I'm fairly sure Rip said some won't reach depth at first but will after a few times.
Haven't heard of resistance stretching. Even though cha usually says hamstrings won't be the cause of not going to parallel, given how insanely inflexible you are I think there's a decent chance it's at least part of the reason. I'd try PNF stretching, which is basically a form of "dynamic" static stretching (yes, that's a contradiction in terms). See
http://www.youtube.com/watch?v=791XXiYzNbE for a hamstring PNF stretch.
I also highly recommend getting the magnificent mobility or assess & correct DVD for dynamic mobility work.
Brag: I can almost put my hands flat on the floor with straight legs.