Yeah the real problem seems to be that the issue was never addressed when the league was created. I've been keeping a list of A-ball guys who could reach AA by June since the start of the offseason, and may have acted differently in the past had I known none of them would be addable.
But I guess I'm flexible when it comes to rules that cover up loopholes/unforeseen circumstances, as opposed to rules changes that are purely cosmetic. So if it's agreed that this is the former, z3r0s' rule could work.
I have another idea though. Every team has 100 'AA prospect dollars' (AAPD) to spend throughout the season. Whenever a prospect is promoted to and first appears in a game at AA, every owner has two days to submit a silent bid of AAPD to a commissioner through Proboards. After the two-day deadline passes, whichever owner submitted the highest bid is awarded the player, and that owner has their bid deducted from their total AAPD pool. In the case of ties, the owner with the higher waiver priority (or maybe lower standings position) wins.