It looks like most of the automation already exists for converting package.xml metadata into the PKGBUILD files and that could be integrated into bloom. It was designed to support extensions to other platforms. From the original Ubuntu it’s already been extended to Debian and Fedora adding Arch could be done as well.
And yes we do have a lot of infrastructure on the ros_buildfarm for dependency tracking and rebuilding appropriately.
It would be great to add support for Arch to ros_buildfarm if anyone is interested in thinking about this I’d suggest starting a new thread at: Buildfarm - ROS Discourse