Ah yes, that would work pretty well! You'd have to be able to guarantee any migrations can run within the timeouts, but at a per-tenant level that should be very doable for most cases. Not sure why I didn't think of that approach - great idea.
I think those SQLite databases are capped at 1GB right now, so even complex migrations (that work by creating a new temporary table, copying old data to it and then atomically renaming it) should run in well under a second.
I might have to try this out now.