What is the best way to maintain a branch based on a mercurial repository?

Hi,
I'm branching of an existing project hosted in a mercurial repository. I would like to maintain a bzr based mirror of the trunk branch in launchpad. I'm wondering what is the best way to handle this since launchpad does not support mercurial branch import (from what I've read in the doc).

I guess I'll have to handle the synchronization myself. How do I do that? I need to preserve the log history to figure out merge issues, so plain file copy is not desirable.