This seems to be the weekend to do all the virtualizers -so I've installed OpenIndiana (Hipster 2021-10) which can use UEFI boot now.
I made my hard disk too small so I created another one and then realized that I have no idea how to find out what the name for the disk is.
Then I remembered -format lists the drives it can see. So:
root@openindiana:~# format -e
Searching for disks...done
AVAILABLE DISK SELECTIONS:
0. c5t0d0 <VBOX-HARDDISK-1.0-36.00GB>
/pci@0,0/pci8086,2829@d/disk@0,0
1. c5t2d0 <VBOX-HARDDISK-1.0-32.00GB>
/pci@0,0/pci8086,2829@d/disk@2,0
Specify disk (enter its number): ^C
root@openindiana:~#
So now I know I can add c5t2d0 to the end of a zpool create incantation.
As I say; hacky, but it works...