Storing (hidden) per-commit metadata

To allow round-tripping pushes from Bazaar into Git, I'm looking for a good place to store Bazaar semantics that can not be represented in Git at the moment. This data should ideally be hidden from the user as much as possible; it would e.g. contain mappings from git hashes to Bazaar ids.

One option would be to store it (as hg-git does) at the bottom of each git commit message. However, given the amount of data and the its kind, it would be annoying to have it displayed by e.g. "git show" or "git log".

Some people have suggested I use the new git notes to store this metadata, but I haven't quite figured out how to add notes that aren't displayed by git log/show and are still propagated along with the revision. Is that at all possible using notes, and are they the right thing to use here?

There also doesn't appear to be any documentation on notes in Documentation/technical at the moment. I'm happy to contribute some if somebody can provide pointers.

Cheers,

Jelmer