These days, I'd build a live system with zfs. It can be done by implementing a geom kernel module that can read an iso file system, and provide the contents of a file as block device (similar to md), but make it read/write by writing modifications to main memory.
Having this, you can burn every zfs-freebsd installation onto cd or dvd with the following steps:
1. look at your storage utilization with df.
2. create a zpool backed by a file, big enough to hold the desired filesystems.
3. zfs send and receive the filesystems on the new pool.
4. export the new pool and construct an iso image with freebsd boot code, loader, kernel, modules and the file holding the zpool.
to install the system after booting the live cd, the zpool replace command can be used:
1. create a new partition on the hard disk. this can be bigger than the cd/dvd.
2. use zpool replace to copy the pool to the hard disk
now you have a working installation without rebooting. the cd/dvd can be removed.
so obviously, this geom module is what I dream about. maybe geom_ghost?