Our project has a notion of dynamic owners that can only be computed by running code. This has to do with MAINTAINER in port file. In addition, we use a MAINTAINERS file, similar to google OWNERS file. While I don't expect phabricator to grow custom logic for us, I'd like an easy way to hook into the the owners application and tell it who owns what and trigger and audit if the owner didn't review the diff.
I'm just going to merge this into T3980, since projects can be owners and that would allow projects to have dynamic membership. I don't plan to implement a separate mechanism in Owners.
An alternate approach would be to synchronize them by pushing changes in after T5873.
Because OWNERS files are common it's possible we may explicitly support them eventually, but I haven't seen other interest in this and don't have any particular plans to pursue it at this time. It is also involved to implement in a general way, because OWNERS files are per-branch but ownership is not.