systemd package has a build dependency on elfutils-devel.
I patched elfutils-devel and performed a scratch-build with “fedpkg scratch-build --arch x86_64”
My new elfutils-devel package is now available on koji.
I would like to perform a scratch-build of systemd, using the version of elfutils-devel I just built.
I see there is a way to chain builds in  but it does not seem to apply to my scratch builds.
Do you know if this is doable?
 Using the Koji build system :: Fedora Docs
Scratch builds cannot be chained because the build results are moved to a per-build destination. You can do chain builds locally in mock, though.
Untested idea: Do side-tag builds work against an srpm or against a dist-git fork? [Update: Yes]
If not and if you want to test on multiple archs/builders then copr is you friend: All packages built in a copr repo are available for builds in that repo (and you can add others). That is well tested