I thought of this after last GURPS Thursday, but since Christopher Rice is at it, I thought I'd throw this out there, no real testing.
Evaluate as free Deceptive Attack.
You can either take the bonus to hit (obviously, if you're striking from behind) OR give a penalty to your foe's Active Defenses. It's simple, though I have no idea if it's balanced. Maybe I'll try it this Sunday, maybe I shan't. They could well have more combat than normal.