FWIW: Travis is working on a build.run API: https://docs.travis-ci.com/user/triggering-builds

You could also ask Travis for a build.run sort of API. If you're a paying customer of theirs, it might be easier for them to prioritize this request than it is for us to prioritize Travis support. With such an API, there's a fairly straightforward path to Phabricator upstream support.

If Harbormaster does not trigger the build, this system may race (Travis may complete a build before Harbormaster generates a target).

I consider this negligible -- Travis build takes at least a minute, because of machine startup times. I've never seen Harbormaster generating the targert taking over a minute.