Can confirm the above listed path does indeed exists.
Maybe bumpspec and try again?

That worked on rawhide and f22 but failed on f23.

Still failing on f23. Can you remove the src.rpm?

I think the import error might have occurred not because it tried to overwrite the srpm, but because the build shipped a 'darkplaces' subpackage with the exact same NVR that was already in /mnt/koji/packages.

Ah, ok. I had intentionally dropped the release requiement but we'll see how it does.