Sometimes we try this situation in the dojo. It's really hard, I think.
What I try is to step out of the line as in the ''the fist stays''-situation, get contact with my hand, move back with the fist of my partner while keeping contact and perform a very short (and painful) version of kotegaeishi. But it sounds more easy than it actually is. And with the guys that also do karate, I don't have a chance yet!
Hope that helps,