For EPEL, modules need to be able to build against modules that are sourced from external repos. This obviously requires work in Koji (tracked in koji#2483) as well as MBS (to be able to be configured to know some platforms rely on external repos for modules and need this configuration instead of doing strange cherry-picking stuff).
I don't know how much of MBS is dependent on it knowing everything about every module, but that is not a sustainable way for it to operate for the third-party packager case (as EPEL is to RHEL, and most people are to RHEL or Fedora).
I second this, as right now MBS is hard-coded to only use one type of repositories. Also if someone is building locally it would be great if more options where configurable from the CLI. (mock configs, alternative git repositories etc.)
Couldn't RHEL streams be imported into EPEL MBS as virtual streams? Then the only remaining change would be the Koji ticket to configure mock to disable '*' and enable the build-required "virtual" streams.
As of Koji 1.28, it is now possible to configure module streams per Koji tag, which should make handling "external" modules possible in MBS without weird trickery.
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