Currently external repos must be merged with the Koji model in mind (treating a build as a unit). Instead, Koji should be able to pull in external repos as-is.
@breilly Is this solved by 'bare' merge mode?
Metadata Update from @tkopecek: - Custom field Size adjusted to None
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1024
Please continue any further discussion there.