so rpkg clone should run something like
git config --add remote.origin.lookaside_cache "http://copr-dist-git.fedorainfracloud.org/repo/pkgs/@realtime/stable/kernel/"

What problem would that solve? I'm not seeing any advantages. The disadvantage of this approach is that it would be much harder to update the URL in case the server changes.

when fedpkg/rpkg introduced namespaces rpms/package, all changes, to avoid annoying warnings, resolved oneliner for find with sed inside.
when packager maintains more packages and for each copr/package having extra config is non-effective.

Note that with rpkg utility (https://pagure.io/rpkg-util), this problem is solved by being able to place rpkg.conf into git repo toplevel (git rev-parse --show-toplevel), which means it is possible to have per project configuration for lookaside cache.

This issue has been unresolved for more than a year, and is going to be closed within a week if no further action is taken. If you feel this is in error, please contact me.
This is a cleaning process suggested by Jay Greguske. Copy of this ticket was already closed in JIRA tracker.