Jorn, I've done some work on selectors for groups, and it's actually quite easy to select a group or sub group for validation, although I don't know the details of your implementation, so I wont suppose to imply anything.
As usual I've uploaded some demo code for you to take a look.
www.janepatrick.co.uk/admin/test/associative-select.php
Hopefully this will save you some thinking time!
Cheers,
Dave
