I guess we might need to attach multiple QMP monitors for this to work
(one for libvirt, one for the rephub). I'm not sure if there is a
fundamental problem with this or if it just needs to be done.
Agreed. You can already query block devices using QMP 'query-block'.
By adding in-process NBD server support you could then launch an NBD
server for each volume which you wish to replicate. However, in this
case it sounds almost like you want the reverse - you could provide an
NBD server on the rephub and QEMU would mirror writes to it (the NBD
client code is already in QEMU).