Right now, koji simply executes scm command inside SCM folder and expects to get src.rpm.. It would be nice if that command would know about build tag, whether it is scratch build or not at least, but generally tag/build information would be great.
Even, better, it would be good to understand any environment variables that could be passed, like the repository git url, for example.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1778
Please continue any further discussion there.