First off, don't migrate things to beta sites. Just don't. 

Ok, if it's *blatantly* off-topic where it was asked, a *shining example to the world* of a bomb-ass question *and* you're 110% certain it'll be well-received where it lands... Yeah, just don't migrate to beta sites.

That out of the way... Generally, *moderators* shouldn't be the gate-keepers for a site. A well-run site with a thriving community will have a host of active users who can close, edit and comment, working to salvage any *worthwhile* questions that come their way. Of course, beta sites are not generally thriving communities with hosts of active, moderating users - see above...

With all that out of the way, all that's really needed to "fix" the current migration system is a message in the "off topic" banner on rejected questions noting the site where the close voters *thought* it was on-topic. Anything more complicated than that is a waste of effort, IMHO - only something like 60% of askers even follow their questions to the destination site anyway, and a much smaller number will ever put any effort into fixing them if they're closed.