I did contribute on the RBDSR plugin (v1). Creating a LVMoRBD would be easy. The other good point using LVM over RBD is the native fsfreeze when doing snaps and co which is not the case with rbd snaps. VM has to be paused (which issues fsfreeze) before rbd snaps.
But LVMoRBD would put all VM on 1 single RBD. Difficult to maintain regarding dev size.
Best would be qemu-rbd. I did use it like 10 years ago over native xen and it did work fine (ensures fsfreeze etc).
There were a tech preview using libvirt with xenserver. Didn't test if it works on upstream version 8.