The big issue of supporting CEPH is that this technology is changing major version every 6 months with breaking changes to its client. So the big question is WHICH client version should XEN support?
I think that the best way to handle this issue is to virtualize the NBD client version of CEPH.
The NBD client doesn't need a specific kernel version (it just works with some performance issue).
In this case XEN can support different CEPH client version by choosing the right client based on the configuration of the SR.