validation of private launchpad branches only works for lp prefix

[jenkins]

Lets imagine I have a private project P and user U who has the permissions to branch the trunk.
With the current state of pbuilder plugin I need to add something following to the jenkins job configuration:

Main Branch: bzr+ssh://<email address hidden>/+branch/P

While an lp project, the plugin doesn't validate this as a valid entry and gives the following error:
Only launchpad registered projects are supported at the present moment.

OTOH, the configuration works even with the error so fixing this would only enhance the user experience.

PS: other option how to solve this would be to make jenkins use the credentials of user U. I however can not use this approach as I need to use more than 1 private user/private projects.