Hi everyone,
I think the probability of someone having an unattached account at an editathon is negligible. Most users that go to an editathon register a new account, and most do not create unattached accounts in the process. I think you're safe to proceed.
Besides, the SUL finalisation is on the horizon (no date yet, but we're making good progress, so the date for the finalisation should be on the order of months away), and once that's done, it will be by definition impossible for someone to have an unattached account. So your solution is future-proofed. :-)
Hope that helps.
Dan