I saved the best for last. The final idea was to embed format information within MIME properties:
Use a registered MIME type, but append extra format information as a MIME property. I did a test on this with Firefox and Safari and it doesn’t screw up the viewing. So for me, MIME properties it is! I would be interested in links to other thoughts on this.
I don't know whether the set of properties one can add to an existing MIME type is fixed -- but if not, this seems like an interesting alternative.