An easy system would be to add a new event "flush_plan_complete" which will put you into a flush() right as the full plan has been assembled, but before any SQL occurs. It could allow you to register more objects for activity, and it would then rerun the flush plan to consider any new changes (since that's how it works right now anyway). How this registration would proceed is tricky, since it would be nice to use the Session normally there, but that makes this more complicated to implement. But then it could iterate again through the new changes and find any remaining steps to take before proceeding normally.
I hope this answer helps you :)