https://launchpad.net/products/backwards/alpha tells me that "No revision control details recorded for alpha" and "If upstream is maintained in CVS or SVN, then you will need to provide us with the location of the upstream branch so that it can be imported into the Bazaar."
https://launchpad.net/products/backwards/alpha/+source allows me to specify a CVS branch, an SVN branch or a tarball. But I already have a Bazaar branch upstream for this release series! It is known to Launchpad. It is described at https://launchpad.net/people/hypatia/+branch/backwards/alpha. It seems logical that I should be able to specify a Bazaar branch to be the source for my release series, particularly one known to Launchpad, but this option doesn't seem to be exposed.
This was marked as a dupe of 31308, which said that it was not possible to represent this state in the db. That's now fixed, but an important ui level issue still remains. The text on the page is as reported by Mary, and it looks like you cannot register a Bazaar branch. In fact you can through "edit source" but the text really needs to change.