Boot an Existing ZFS From a FreeBSD LiveCD
Because I ran gpart bootcode on the wrong partition of every replacement disk I swapped and because my motherboard is incapable of finding an EFI partition, I basically bricked my FreeBSD NAS / gateway.
It took me a ridiculous amount of time in order to find how to boot into an existing ZFS FreeBSD installation from a FreeBSD LiveCD (I used mini-memstick).
Finally, in a 2014 thread from the FreeBSD questions mailing list, someone mentioned the magic invocation, and from there I deducted the remaining parts.