Regarding the Xen for SL 6: I note that the xen-4.1.2-8.fc17.src.rpm from the bleeding edge Fedora 17 test repository seems to compile well using "mock" for Scientific Linux 6: I don't have a good host to test it on, but backporting those might be a good way to provide access to this.
Building customized relese 3.x kernels to get the published Xen patches integrated, that's a whole different ball of wax.
