Can multiple KVM instances run well sharing a single btrfs filesystem? That's what's so great about containers/jails/etc over full virtualization -- there are no problems sharing the same filesystem.
Maybe you meant snapshotting using LVM? I've done that before. It works fine, and you can boot different machines onto different snapshots of the same filesystem. For some reason though my attempts to use LVM usually end up in tears.
Or maybe you're prosposing doing everything offline and never booting more than one machine on the btrfs filesystem. That would work but I'm not sure what btrfs would buy you over using a more mature filesystem on LVM. Ultimately, I'm skeptical the minor benefits would be worth the downtime and effort it would take to do set that up!