How to contribute a fix to bazaar, as light weight as possible

I would like to contribute a fix for a bug in bazaar. I do not want to do a full branch because it takes a heck of a lot of bandwidth. I really just want to make my changes to the latest revision.

Is there an option to do a light weight branch, ie only get TIP and make my change and push it to a registered branch on launchpad.

Or do I have to use light weight checkout, unbind and push to a registered branch? This didn't work for me on the current bzr trunk. (unbinding my local checkout)