'bzr missing' is working on the Branches, and there isn't a separate "branch" for a lightweight checkout. It certainly could be updated to use the WorkingTree.last_revision() instead of the branch's last_revision. It would take a little bit more work to detect when it is a lightweight checkout, and change the default "other" branch.
It actually *is* working for your lightweight checkout in a sense, but it is comparing your branch against the parent branch. Which isn't really what you want.
'bzr missing' is working on the Branches, and there isn't a separate "branch" for a lightweight checkout. It certainly could be updated to use the WorkingTree. last_revision( ) instead of the branch's last_revision. It would take a little bit more work to detect when it is a lightweight checkout, and change the default "other" branch.
It actually *is* working for your lightweight checkout in a sense, but it is comparing your branch against the parent branch. Which isn't really what you want.