OK, even accepting that the actual process of migrating questions [should be left to moderators][1], the current user-interface flow is suboptimal. So I suggest adding one more option to that dialog:  
This question…

* is off topic
* belongs to foo
* belongs to bar
* **should be migrated to `_____` by a ♦ moderator**

You could do this both for the *flag-for-migration* case and for the [*vote-to-close* case][2]. The technical effect would be the same in both cases: a moderator would have to migrate the question. But the user experience would be different:

* Selecting an option and entering text into a field feels like going *forward*, in contrast to the *back* or *cancel-and-flag* steps required otherwise.
* Going forward feels like one did the right thing, whereas having to go back feels like having made an error.
* This approach also reduces the number of clicks required to get this flagged properly.
* Having only to enter the name of the target site, instead of a full sentence, caters for <strike>lazy</strike>busy people.

Seeing as this question here currently has [meta-tag:status-declined], should I file a new question to get some attention for this proposal?

  [1]: http://meta.stackoverflow.com/a/96345/188688
  [2]: http://meta.stackoverflow.com/q/86928/188688