After converting my old svn repositories to bzr, I was left with a few other projects that had been in bzr all along that I wanted to integrate into the larger bzr collection. In this e-mail thread in one of the bzr mailing lists. I found a technique for doing this.
Currently, the way to do it is to set the base to 0 explicitly. E.g. "bzr merge -r 0..-1 OTHER_BRANCH".
I'm such a bzr novice that I don't even know for sure exactly what this means but it seemed to work.