The reason that I don't like this (if I'm understanding your idea) is that the pilot that drops the trip loses flexibility.

In your example, what if "pilot A" wants to still fly (get paid) 80 hrs? He may be dropping the trip because he needs a specific day off, whereas he'd pick up something later in the month. If he has that 15 hr shadow additive, he wouldn't be able to get back up to 80 hrs in your example (as far as opentime pickup.)