When I do an off-topic VTC and select "belongs on another SE site" the box to select which site it should be migrated to only contains meta.aviation.SE. Do you need to have a certain amount of rep in the other site or something?
The short answer is "We haven't set up any migration paths yet - the lazy-ass moderation staff should get to work on that!"
The longer answer is we've been kind-of ignoring the issue since we really don't have a lot of migration-worthy questions that would get kicked to another site as good/answerable questions, and when they come up they can be flagged and mods can tidy them up (fixing comments and such) before we send them to a good home.
If you make a good case for specific migration paths (Travel and Space both seem like good candidates) here on meta we can poke the developers to set them up, but unless we're seeing a good trend of accepted migrations to another site we can probably keep working with the "Flag-and-we'll-deal-with-it" migration system.
For reference there have been 2 migrations in the past 90 days: 1 to Travel, and 1 to Engineering - it's not like the volume is killing us :)