Back in 2012 there were patch proposals by M. Mohan Kumar  to add support for QEMU's 9p proxy driver. These seem to have been forgotten. I couldn't find any more chatter about them on the lists and they apparently haven't been applied.
Did I miss something?
It would be nice to add support for this driver.
The original author has never sent a new version of the patch and nobody else from the community seems to be interested in this topic enough to implement the feature.
I think the focus shifted from 9pfs to NFS over virtio-vsock, although this is not yet implemented in libvirt either.