Thanks for the reply, @calbzam. If I understand you correctly, this would give me the encoding a file had in a particular revision. I would then have to somehow track that across invocations to find the revision when it changed, right?
Also, I need this for all files in the repo which match a specific extension wildcard. It seems a bit overwhelming to do this by revision & file "cross product". Is there perhaps a way to get a changeset's diff in textual or otherwise script-processable form?