Maybe a maintainer can mark a repo as orphaned and others can bid to adopt it using their karma? Then the maintainer has a list of people to talk to about the future of the project... just helping make that transition easy would be nice.
A simple project setting that says "Advertise as orphan" would suffice. Then a banner would appear on the page saying "This project is looking for a maintainer!" and have something akin to an apply button where you might add a short comment about your interest.
With the "Login with GitHub" feature now available, this could even be a third-party service.
