How to boot multiple FreeBSD systems (GPT, ZFS)?

[Warren Block]

On Sat, 22 Feb 2014, Jens Schweikhardt wrote:

> hello, world\n
>
> in the good old days of MBR the FreeBSD bootblock would let me
> select one of several root partitions to boot from with a prompt like
> F1 FreeBSD
> F2 FreeBSD
> F3 FreeBSD
>
> Those times are gone; the new kids on the block are GPT partitioned
> disks and ZFS. I've read many man pages and articles on the net on using
> gpart, gptzfsboot, bios-boot partition, separate mini boot partition,
> setting up a root on ZFS system, and others. The part where I'm having
> difficulties is: which program lets me do the selection?
> ***No, I can't use grub or grub2.***

Syslinux?  Probably not, but thought I'd mention it.  If choosing 
after boot is adequate, see sysutils/beadm.

> Can anyone outline how to partition and install a system that lets
> me select at boot time which of several ZFS root file systems to boot?
> I'm fine with the big picture and can figure out the details myself.

Does the pool/filesystem choice need to be presented by a menu, or is 
typing in an already-known value good enough?

Consider also VMs with VirtualBox or bhyve, using zvols for disk.