An readily-available "escrow" service for source-code would be interesting. I know this happens with corporate contracts, but an easy-to-use public version might be nice.
The source would remain closed until certain circumstances happen, when it would be released automatically under a chosen open-source license.
The death of the author is the obvious one, but (less morbidly) the developer could say "if I don't update the code in 10 years..." or "if I don't log in and hit the "Renew" button within 5 years..." then open the code under GPL. The criteria could be defined by the author themselves.
They could remove the code from the service later, but that's their decision. The thing is that they're not transferring any ownership or rights at that stage.
It would mean that there's no need to persuade a disinterested author to hunt through their attic looking for a disk, assuming they can be persuaded to put their code on there while they're still actively developing.
Does such a thing exist already? Would any devs reluctant to open their code sign up to such a service? Just a thought...