if there's a way to tell whether an 'away' player pressed the 'sit-out' button or just lagged, could they be treated differently? i.e. harsher penalty for the 'sit-out' guy. probably too complicated though.

Actually, I think the opposite. If I'm shoring up and waiting to be all-eighted, I should be able to click "away" and run and get a cup of coffee, and then come back and click 'sit in' (like I can do now). If I was punished for clicking 'sit out' I'd just let it lag out, making everybody else mad at me for slowing down the game.