you can install packages like this:
$ dnf --disablerepo='*' --repofrompath brt,https://fedorapeople.org/groups/modularity/repos/base-runtime/26/ --enablerepo=brt install ...
I though about that solution. But I'm not sure if it is okay. because in case you will have in test installation of something like "microdnf install something", in "rpm" type it will fail, because no repositories will be there and it will not be clean where are sources for installation, not stored in configs.
But this rpm part is little bit problematic and does not have clear solution. You've probably talked about "nspawn" and for this nspawn solution I can remove to creating these repositories and it will work well, because repositories are stored in change root directory.
Metadata Update from @jscotka: - Issue tagged with: Prio0, RFE
it is solved in nspawn scheduler, it uses your suggested solution
Metadata Update from @jscotka: - Issue close_status updated to: upstream - Issue status updated to: Closed (was: Open)