This question has been answered in the answer section, after six months of investigating :P
I've freshly installed Ubuntu 22.04 and used the ZFS+LUKS full drive encryption option from the installer.
I've been searching around on the web for a way to configure the drive to unlock automatically on boot when a usb key is detected (LUKS unlock key on the USB stick) instead of entering a decryption passphrase every time I boot the system.
I've found tons of guides, but none of their instructions seem to be applicable to the way 22.04 sets out the partitioning structures on the drive when using both ZFS and LUKS.
The closest guide i'm following is here: https://tqdev.com/2022-luks-with-usb-unlock
Everything seems to go well, until I get to the point where I have to add the key to the LUKS drive (step six) - because two drives are listed as LUKS drives.
root@bob-home-linux:~# sudo blkid --match-token TYPE=crypto_LUKS -o device /dev/sdc2 /dev/zd0
I'm not sure how to proceed from here!