I don't know how it works exactly, but would it be possible to have some sites other than Cryptography Meta in the "This question belongs on another site in the Stack Exchange network" list for closing as off topic?
For non-beta sites, you can request that migration paths be established for certain other sites. In these scenarios, if enough people choose the same site when voting to close, the post is automatically migrated. Since we are in beta still, we don't get that feature yet. So, for now, the best option is to choose other and specify the site. Then a Mod has to migrate it.
Putting it into pictures(mainly for reference purposes in case new users stumble over this question)…
Click the close link
Select “off-topic because…”
Select ”Other”, and add a comment explaining what is wrong. Finally, don’t forget to mention which site the question should probably be migrated to, and then click “Vote”… done!