As of the latest build of TDS, 1.2.2268 you can now send ? bitizens to any level. In fact the game now gives you a pop-up encouraging you to try to unlock scenes, and another pop-up telling you that you will get a buying frenzy for delivering them.
Looks like you can send them to any level then, and they'll buy an item and take a minute off the restock timer as usual. This works to your advantage since you can either send them to a level that needs them or to a level that'll unlock a scene from the character/store combo.