The main issue is the need for a reproducible build, which is hard to do because macOS doesn’t have docker and no one (outside of Travis-CI that I’m aware of) has automated use of VM’s with checkpoints to emulate a “clean starting state” for each build. This is a crucial step for building packages in my opinion, but AFAIK (it could have changed) macstadium.com only provides hosting on mac minis, not the automation for repeatable jobs. Travis-CI does provide this as continuous integration, and I believe some people are using (abusing?) this to build Homebrew bottles @scpeters might know more about that then me at this point.
If you’re interested in trying anyways, you’ll want to catch up on the existing efforts:
Those two links are the best summaries I can find, but efforts so far have stalled, as just keeping it building from source on macOS with Homebrew has become of a burden than the community appears to be willing to bear.
In order to be integrated into the “ROS buildfarm” (by this I assume you mean build.ros.org), we’ll need a way to provision machines, generate artifacts on a per package basis (bloom does this for debian and fedora packaging), and extend the build farm itself to create the necessary jenkins jobs to actually execute the packaging on the build machines and upload the result somewhere. Once you get to this point, @nuclearsandwich and/or @dirk-thomas might have more to add.