I gotta agree with John here... Your objective is noble, but doomed.
Background: The pressing need for a SO dumping ground
For its first year of existence, Stack Overflow faced a constant battle between users who wanted a site dedicated to programming questions and users who liked the format and wanted to use the site for... any topic that interested them.
UserVoice, the blog, and Stack Overflow itself were filled with the anguished and often angry cries of users who had been slapped down, told that their questions about harddrives, games, office politics, and favorite peripherals weren't appropriately "programming related".
It got ugly. Really ugly. Much uglier than any discussions surrounding bits of text should ever get. When a group of users becomes so used to being called "fascists" that they actually petition the site for a badge recognizing this dubious distinction, that's too ugly. And as the site grew more and more popular, it just kept getting worse...
Super User: a light at the end of the tunnel
When Super User was announced, there was one criteria for the questions asked there:
If your question has to do with computers, it will be allowed there.
This was a welcome announcement for many. Even those of us who didn't plan on actually using the site, as it signed an end, at long last, to the tiring arguments, close wars, and bad-blood between users who wanted a programming site and users who wanted a "computer things" site. The ability to migrate questions meant that an off-topic question couldn't be easily re-opened on Stack Overflow, but could still be answered - a rare win-win for both camps.
A site about everything is a site about nothing: the ongoing Super User lock-down
Almost immediately, the original definition for Super User was challenged, as users of the new site attempted to come up with more specific criteria for questions. By the time the public beta opened, the definition had already changed to exclude questions relating to video games or consoles.
Still though, the rules for migration were simple: computer-related but not programming-related and not game-related? Move to Super User! And that's pretty much the only rules I and many other SO users bother to remember even now - although I do try to follow some of the discussion here on Meta, I wasn't really aware prior to reading this question that SU now has an entire bullet list of topics that shouldn't be asked!
Conclusion: The fresh paint on your barn wall is not my responsibility
Bad migrations will happen: because we aren't SU users, because we're not continually checking your FAQ for new taboos and because we're sick of fighting with people who just want answers to their computer-related questions. If you don't like it, I don't blame you - but posting a request here isn't going to accomplish much. Remember, SO was programming-specific from the very start, and still gets a never-ending stream of off-topic questions, many of which aren't even computer-related.
So you have your work cut out for you. John's suggestion for a special "migrated questions view" could work... if there were enough engaged 10K users on SU to actually make use of it. As-is, I suspect such questions will continue to be the responsibility of you and your fellow moderators.
Perhaps you could use a tool that would let you reject migrated questions...