Have Emulab install and use ZFS on the FS node.
I think this is largely a documentation issue--having people setup ZFS on the fs/ops node with guidance for how to size FSes, etc. I have the rudiments of this already in /users/mike/zfsforfs.txt. There may be some tweaks to the install and elabinelab code as well.
I had to manually convert all the Cloudlab clusters over to ZFS which was a pain, and I was reminded of this again when I setup the Onelab site (ZFS clones are needed for the admin MFS) and yet again the other day when I was updating the geni-lib jail stuff (clones are used to spawn the jail).