Thanks guys. From what you told me here and some subsequent googling, as far as I gather, this is probably Dean's escape but maybe with the elbow on the floor rather than on the opponent's body.
Here's a pic from a sherdog thread of what Ryan was calling the RG escape-
rogergraciedefense.jpg
And here's DP vs Zombie, 19:40:
http://bashtube.ru/video/frame/937f1...4a370a7af.html
He didn't grab behind the thigh though, the free arm was fully occupied
I also found what seems to be a gi variation of what we're talking about here -
http://bjjr.ru/techniques/intheguard...e/_default.htm