Content generator backends would benefit if they'll be able to 'reserve' NVR before they'll start generating content, as it could be taken meanwhile by some other tool. Or at least to have deterministic input to process. It could be modelled in similar way, how init_build works for rpms.
init_build
Proposal: PR #1464
Commit bf394684 fixes this issue
Commit 553871be fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1463
Please continue any further discussion there.