I tried a few (three? four?) years ago and there were iirc three
showstoppers back then. I'm sure I wrote them down somewhere, but I
can't find it right now. 🙁 But I think I remember one:

It iirc (and afaik back then) was not possible to build packages and
test them before their publication. Is that possible these days? I
occasionally do something stupid, that's why I fire up the x86_64 builds
in qemu locally these days before publishing a build.

TIMWTDI

1) If you are using pull request then you can use Packit https://packit.dev/
Packit build the code in PR in new Copr project and links results from PR.

2) If you are not using PR and want to share a results before you push it, you can create project, e.g., thorsten-testing, and configure it that is requires some other repo (your upstream project) and all build in thorsten-testing can use package from some-other-repo and your builds in thorsten-testing will not impact some-other-repo.

Generally speaking: creating project is cheap task and you can create/delete project on daily basis.

Seems it's somehow possible these days:
https://docs.pagure.org/copr.copr/createrepo.html
Is that scriptable? I'd prefer not to do this manually for each build.

copr-cli modify --disable_createrepo true

copr-cli regenerate-repos

But I would use one of these options above. The createrepo is originally inteded for larges projects like KDE, Gnome. Where you have to builds lots of package and it can takes days it you get all packages built. And during that rebuild the repo can be broken.