I wonder if something like that should be built on top of GitLab now that you have migrated to it. GitLab CI supports multiple runners for a single job, so the workflow might be that you merge a merge request which modifies PKGBUILD/.SRCINFO, the CI picks it up and based on labels/tags dispatches it to different runners running possibly on a completely different architecture for building. The runners then send the artifacts back and the job publishes them. The nice thing about this is that most of the parts are already there handled by GitLab itself, you just have to draw the rest of the owl.