Building is just one part of this. We also have to think about source control and test infrastructure.
For read access, a git repo is all or nothing, so I presume we'll need a system where we handle embargoed code in a separate, locked-down git repo (not just a branch) and merge that in after the embargo lifts. Can we make this work with gitblit? Can we make such a workflow tolerable in centpkg?
Similarly, with test infra we'll need mechanisms to test the embargoed builds without making them visible.