I think, that Guidos way is the most successful one. We should no try to be too clever:
The easiest way to customize (field only) behavior TTW, I think, is really to disable the behavior and re-define the fields TTW. This should be especially ok for dublincore fields, because all dexterity objects provide dublincore interface. The current exception for TTW customization are timezone aware datetime fields, because timezone awareness is hardcoded into the behavior (and would need more work).
The easiest eay to customize behavior in python code, I think, is Guido's way of inheriting the original behavior schema and adding your own requirements on top of it. This would require some extra GS or setuphandlers to swap the behaviors and would bloat the behavior selection UI, but those steps can be made easier later.