We could add an 'I did not work out today' button on the workouts section, to indicate that the day was, in fact, a zero, rather than just a failure to fill in, or we could do it as you describe, and give a 0 by default.
On the weight graph this would be possible, yes, but it makes more sense to show the trend between your last and current point - if you went from 220 to 210, it wasn't overnight, it was via 211-219,
We'll throw the idea around here a bit and see what we come up with - any other thoughts on the matter?
Thanks,
Maxwell