I propose to change the workflow so that a closure reason is required after suggesting migration. Namely,
Click "close" or "flag".
Click "off-topic".
The dialog looks just like it does today, with one of the choices being
◉ This question belongs on another site in the Stack Exchange network
Choose a migration target site.
The dialog looks just like it does today. (I'd add emphasis that it needs to be a high-quality question.)
Specify a closure reason.
Flagging > Closing > Off-Topic > Migration > Closure Reason
Why is this question off-topic for insert current site name here?
◎ Standard reason A
◎ Standard reason B
◎ Other (add a comment explaining what is wrong)
Vote to migrate Cancel
The migration flag is cancelled unless a closure reason is given.
One disadvantage of this proposal is that the thought process is a bit backwards: normally, one would classify a question as off-topic before suggesting migration.