I would especially like to see a cryptographic mechanism enforcing such middle ground. That is, a mechanism that would allow law enforcement accessing the private key or plain text, but only with some inescapable side effect which would hinder abusing that power. That may be producing a cryptographic "proof of compromise" for the person being spied on, or the spying being publicized, possibly with some delay, or something else. I am searching for an analogue of having police show up for a home search, which would notify both you and your neighbors, and cannot be done sneakily and by-the-book at the same time.

Are there some interesting candidates for such a mechanism? At first is sounds like a long shot, but there are cryptographic mechanisms achieving unintuitive results, so it may very well be possible.