You’ve (or @bee has) already blogged about this before: http://blog.beeminder.com/catchup/
But I’m sure revisiting the topic would not hurt.
I suspect the solution might lie in meta-beminding. Ideally this would be automated[1] and the second goal is automatically populated based on inputting data in the first.
Main goal: Exercise, where you log the distance you run.
Meta goal: Go for a run 6 times a week. (automatically populated when you enter the main goal data.)
So you can have a 4 day buffer on your running goal, but you still need to at least go down to the end of the driveway today (even if it’s raining) to maintain the habit.
[1] - I’m surprised this is not already implemented as it seems like an easy way for beeminder to increase the amount pledged.