We're getting questions in our private beta community that are very general to the entire platform and should be asked in main meta. This brings up the idea that a "reparent" tool to migrate questions would be really dandy. For starters it wouldn't even have to leave a breadcrumb trail on the site it came from, I'd say they could just be flat out moved.
The feature could be added one step at a time:
1. Let the owner and/or site admins change the site parent ID on questions.
1. Add a redirect so that URLs for question IDs where the question ID has changed sites won't break because the site name will update (benefit of having postings in a single table across sites, there will be downsides later!)
1. Add an entry to the post history showing the reparent event took place.
1. Add stub posts from the source site linking to the new home for the sake of late-comers.
We have no plans to implement anything at like the migration feature on SE — once question deletion is implemented it's a simple thing for someone to manually move a post with cut and paste.
Responsible folk within the communities should be nudging question posters to ask on main meta if necessary.