How about xenbits.xen.org/<subproject name>/staging/ for the staging trees, if there is consensus to do this.
However, if we allowed linux.git and xen.git in xenbits.xen.org/<subproject name>, it does increase the risk of creating a permanent fork (i.e. the main concerns raised by IanC, Stefano, etc.)
True. But this is always a risk (like with mentioned XenRT), even without staging trees in subprojects. To me it is more a matter of maintainers' and community discipline...