OK, looks like there's already a good way to do this:

1. Flag the question

2. Select "It should Be closed for another Reason"

3. Select "Off Topic"

4. Select "It belongs on another Stack Exchange Site"

Then it gives you a choice of a few alternatives which seems to be automated based on keywords in the question or something.

OK, looks like there's already a good way to do this -- but not on a beta site!

1. Flag the question

2. Select "It should Be closed for another Reason"

3. Select "Off Topic"

4. Select "It belongs on another Stack Exchange Site"

Then it gives you a choice of a few alternatives which seems to be automated based on keywords in the question or something.