How about we put a checkbox in the
close as off-topic dialog box that will let you input a comment (as if you had flagged
other), and then after you hit the vote-to-close button it will auto-flag it
other with your comment?
Of course, this checkbox would be radio box option with the other five, so it couldn't be chosen simultaneous with any of those, and of course, you could still not choose it and just close as the generic off-topic.
This will save clicking out of the dialog, and then having to click into the flag dialog.
Edit: For those who think the process should be slow to discourage migrations, I'd like to point out that that isn't a real solution. If we have a problem with migrations (which I would argue against) then solve that problem, don't make the site slower to discourage that.