Technically, the existing EPEL 10.0 is based on a cs10 snapshot. Could we make that accessible to people to use for this purpose?

Yeah, it's available here: https://infrastructure.fedoraproject.org/repo/centos/centos-10-snapshot/
but not sure we want to expose that to every mock user in the world. ;( Although I suppose it could be disabled by default...

I understand the desire for this, but it's complicated. RHEL 10.0 doesn't exist in a public state yet. The epel10.0-build tag is currently using a snapshot of CentOS 10 until it can switch to the real RHEL 10.0. Ideally we aren't building a lot of things in this tag, and most packagers are shifting their focus to 10.1 via the epel10 branch. New packages (the biggest concern for installability problems) should be 10.1 only at this point, which have the bodhi installability check and work well with a local mock --postinstall. Any updates being shipped for 10.0 now should be things that are really necessary for an existing 10.0 build. If you absolutely need to do this, you can ask koji for a mock config by running:

koji mock-config --arch x86_64 --tag epel10.0-build

I'm ok with this level of friction because I agree with Kevin that we don't want every mock user hitting the snapshot directory.

You can also test installs on CentOS 10 by overriding the releasever variable. This isn't perfect because it doesn't change the CentOS packages available, and could lead you to a mismatch between 10.0 and 10.1 libraries, but I don't think we can solve this since CentOS doesn't actually have minor versions anymore.

dnf --releasever 10.0 install $pkgname

I understand the desire for this, but it's complicated. RHEL 10.0 doesn't exist in a public state yet. The epel10.0-build tag is currently using a snapshot of CentOS 10 until it can switch to the real RHEL 10.0. Ideally we aren't building a lot of things in this tag, and most packagers are shifting their focus to 10.1 via the epel10 branch. New packages (the biggest concern for installability problems) should be 10.1 only at this point, which have the bodhi installability check and work well with a local mock --postinstall. Any updates being shipped for 10.0 now should be things that are really necessary for an existing 10.0 build. If you absolutely need to do this, you can ask koji for a mock config by running:

koji mock-config --arch x86_64 --tag epel10.0-build

I'm ok with this level of friction because I agree with Kevin that we don't want every mock user hitting the snapshot directory.

You can also test installs on CentOS 10 by overriding the releasever variable. This isn't perfect because it doesn't change the CentOS packages available, and could lead you to a mismatch between 10.0 and 10.1 libraries, but I don't think we can solve this since CentOS doesn't actually have minor versions anymore.

dnf --releasever 10.0 install $pkgname

Fair enough, and thanks - I didn't know about koji mock-config before!

This issue has been migrated to Fedora Forge:
https://forge.fedoraproject.org/epel/steering/issues/321

Please continue any further discussion there.