But that only works in one direction, no? So it works if you can develop your single repo, but it doesn't help you when you depend on other projects.

I think the best approach would be to have bidirectional links between the projects (if A needs B, then A has the stable version of B and vice versa). The point in that setup would be that "upstream" projects can notice when they are about to break tests in "downstream" repos and act accordingly.