I don't understand why the build config is used to generate metadata for the trigger object. Why not use the trigger name, which is required to be unique project-wide anyway?
Even if users have issues crossing wires when their GCP projects and github commit status contexts collide, I couldn't image it being a motivation for doing it the way it is currently.
Currently the only option I see is to set up a cloud function against the cloud build topic and manually update statuses that way.