I went back over the rules of what you can do while grappled and Thomar's right, there's no rules for going prone short of pinning someone, and that actually doesn't explicitly say the person being pinned is prone.

I think my previous post allowed me to put right in my head what I'm looking for. It would basicly be an addendum to the grappling rules with a feat tree to back it up:

Throw your opponent:
If you win at a grapple attempt you can then make another immediate grapple check, if you are successful you can throw your opponent. This puts him prone in the square of your choice, doing your unarmed damage or 1d6 bludgeoning, whichever is more. Your opponent is also prone, though he can make a tumble check DC equal to your grapple check to be standing, and half the damage.

If opponents try to move through the square with a prone opponent from this maneuver that square counts as two squares that doesn't provoke an attack of opportunity.

I have one feat to add to this, and I'll probably come up with some more, but as a starting point, how's this look?