George S. Ledyard
I've trained in a couple of tactical ground defense systems but bar far the best training I've seen for defense against multiple attackers while on the ground is the Systema ground work. It's based on their fundamental principle of constant movement. It is the only system I have seen that would really help you much if you were on the ground in the middle of a group of folks trying to stomp you.
While I don't want to derail the thread with THAT discussion, I'll just say that I don't share your opinion, George.
People can PM me questions or flames, I won't clutter up the board with the inevitable go nowhere debate with the folks that want to defend the faith.