For a long time, Red Hat engineers have dropped public RPMs onto people.redhat.com. Now that CentOS is a more official part of the family, it seems like an obvious idea to me, but why not create a "centos7-devel" branch that is public work that is intended to go into the next upstream update?
Several of the existing repos like virt7-testing and atomic7-testing could simply be folded into this repo.
As well as these "hand built" RPMs: http://people.redhat.com/lnykryn/systemd/ http://people.redhat.com/~rjones/libguestfs-RHEL-7.1-preview/
And I'm sure others.
(I wouldn't be surprised if this has come up before but I couldn't figure out any keywords that would find previous conversation on this topic from the archives)