Automatically build PPA from launchpad branch upon commit

Is it possible to have launchpad build a PPA directly from a launchpad bazaar branch without having to upload the source using dput? For example, I would like to have a PPA that represents the latest version from an unstable trunk. Whenever a commit occurs on that trunk, launchpad would automatically use the latest source to kick off a build of its PPA so users can test out the latest changes. This build can occur upon every commit or possibly at some regular interval after trunk has changed if building upon every commit would be too resource intensive.

The only way that I saw to keep the PPA up to date is to manually check out the code to the local computer, make a source tarball, then upload to launchpad using dput. This seems redundant since launchpad already has the source and it seems tedious since developers would have to do this upon every change to their branch.