Extra metadata in the commit?

Hey all,

I'm working on a hg<->git bidirectional tool using Git as the communication protocol, so there is a bunch of Hg metadata that I need to keep in Git to ensure I can convert commits created in Hg back into the exact same objects after they upload to the Git db and then come back down. This means storing explicit rename info, branch names, etc somewhere without messing up Git clients that want to work on the same repo. So, I can keep this data in the commit message, but I thought it would be cleaner to keep it as extra fields in the commit object before the "\n\n" separator. Before I go too far down this road, is there anything horrible that might happen in any of the core Git tools if extra data is there?

Thanks, Scott