dave, I think you "just" need to look at the consequences of being late, and what those mean to you, and make the decision on a case-by-case basis. That's more realistic than the punishment anyway. All of life is a trade-off, if you think about it. "If I do it with x bugs, then..." "If we ship before y is done, then..." Then you are prepared for those consequences (or not). That tells you whether to keep going.

But I'm in the corner for shipping earlier. For our app, Freckle Time Tracking, we got the time entry interactions really, really done well, but the rest was very beta/alpha. We decided that we wanted to focus on the single biggest, most important interaction. We're still polishing the rest.

It is, of course, an unending task, but you have to use logic & foresight to decide where the intermittent goal posts are.

Creating a punishment for yourself for going over a timebox is not gonna do that hard, human thinking for you :)