Apparently, we cannot flag a question as belonging to other network during beta:
What is the recommended way to go in these situations?
If a question is off-topic here, just close it with the generic off-topic reason. Our scope isn't set in stone yet, so we want to keep things on our site in case they need to be reopened. If a question owner - not just any user passing by - really wants their question migrated, they can raise a custom mod flag and a CM will take care of it. (Though if the question didn't get any answers, it's easier to just delete it here and ask it somewhere else.)
Once the site graduates, we'll get migration paths to other SE sites and we'll become eligible as a migration target from other sites.