Interesting. I hadn't considered that possibility. I'm more familiar with hg than with git, but from what I gathered in #mercurial, it should be possible to write a combined extension + a commit hook that:

* allows people with appropriate permissions to lock and unlock files

* sets locked files to read-only on pull from the canonical repository, so that users are warned beforehand that the files are locked

* rejects commits that modify locked files except for by the person that holds the key

There are a bunch of issues that need to be worked out, but I think the problem is tractable. Nice, thanks for the idea!