How about a standard entity "OSI perhaps?!", that commits a file to an early stage of the repository (could this be automated), who then cannot / will not give approval for a relicense?
Relicensing still can be done, just keeping that file out. (and reimplementing it the same but with new code, if it was really needed for something important)
Love the idea but the thing is, if it's just one file then it's probably easy enough to work that contribution out of the repository.
It is quite fun to try and think of ways that this could work though. Perhaps a bot that code-paraphrases (paracodes?) every accepted PR. Or maybe there's some crypto magic you could do to make the only option a clean room rewrite.