There are at least a couple of impediments to doing what you ask. Migration is a form of closure for a Question within a Community, at least in the sense of making it unanswerable there. Questions with bounties are immune from closure so that no one is deprived of the opportunity to compete for the bounty.
Also your request takes the form of transferring the bounty with the Question in migration. This amounts to transferring reputation points from one Community to another. I don't believe there is any analog for this in StackExchange software functionality, and purists might easily object to permitting such transfers as the Communities are expected to be separate missions. A mature Community and a private beta Community present an even more compelling picture for separation.
But I do find that the idea is novel.