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.
Have to agree with you there George, never thought I'd have so much fun with three boofy blokes trying to stand on my head.
As an aside I asked a security guard friend of mine who used to do a lot of pub work. He made the point that he never wanted to hit the ground because of the reason that he was an easy target for any one. Different scenario than the study but his aim was restraint in the situation where numbers may be against him. Generally though the motivation to resist was generally easily subdued. He is about 110 kg (240 lb) so that helps.