[Libguestfs] Using virt-sysprep and virt-customize with LXC containers: is it possible?

I'm setting up a system to run LXC containers with ZFS on Linux (Ubuntu 14.04). I intend to set up a base image which I'd like to clone to create other containers from. virt-sysprep and virt-customize seem ideal for the task: I obviously could mount and work with the rootfs from each container directly, but the libguestfs tools have a bunch of pre-existing recipes that would surely be much better than what I'd come up with on my own. Unfortunately, I can't seem to find any way to tell the tools to work directly on an already mounted directory (from a ZFS subvolume) instead of a disk image. Am I missing something or is this currently not possible? Thanks in advance, Daniel Miranda