Just as a point of interest, I never did get Ghost to work properly with FreeBSD. If you do manage to get this to work, you'll likely have to run growfs in order to grow the available disk space.
I've just recently started playing with LVM2 on a Debian NAS device I have here, and although I was originally very hesitant, I have to say that the combination of md devices (not to be confused with FreeBSD's md's) and lvm's is quite a refreshing change, and it's a pity it cannot be used under FreeBSD. Instead of being stuck with just /dev/ad0, you could make a pv (physical volume) out of /dev/pv0, then add it to your volume group, /dev/vg0. Once there, then create a logical volume, say pfsense, /dev/vg0/pfsense, and install there. Run out of disk space? Add another drive, create another pv, add it to vg0, run growfs, life goes on. :\
Ah well, please ignore my postulations.