I was going to post this very same question/request in here. Based on Jeff's comment:
"prove it with data of existing closed questions"
I ran into two questions yesterday that could have been answered on other SE sites rather than outright closed:
This would have been a good question for Graphic Design or UX:
web 2.0 web design
And this would have been for Graphic Design:
While I appreciate the intent of keeping the list of 'blessed sites' one can migrate too short, it is probably backfiring now that there are so many SE sites out there. The fact that I CAN'T pick those sites to migrate made me think...I couldn't figure out where the option may be. I finally flagged it and added some comments that it should be migrated, but the questions where still closed.
So, yes, one can add custom comments to a flag to get it migrated, but if those two questions are an example, people aren't using that method.
If the SE network is going to grow at the pace it is, it'd be great to have more of these questions get to the appropriate sites rather than fall off the radar as closed.
What if there was a 6th option to the main 5 of 'other' and, when selected, either a comment field or a DDL of the other options could be presented.