This is now not needed due to the moderator "migrate anywhere" option. So just flag the question and there's no waiting.
I think it would be useful for all (for a given value of all) users to be able to see which migration paths are currently open for a site.
NOTE: The following was a subsidiary idea
Further to this it might also be useful for 3K+ users to be able to vote on which sites occupy the available slots. Jeff has stated here that he doesn't want more than 5 migration targets to be shown and that the 1st is always the site's meta. So the votes would be on which sites occupied the remaining four.
This could be a dynamic thing with the current top four sites being shown (what ever they are) or a periodic thing where the sites change only when one reaches a certain threshold of votes.
There should be a restriction of 4 votes per person (one for each slot).