The ZFS install instructions want to boot from a live ISO and then install all the ZFS stuff into the live filesystem, then do an install from there. Because of various license problems with ZFS it has to built from source. Which seems to work except for one little problem: it builds for a newer kernel version than the one that boots from the Devuan live. 😕
So now I have to figure out how to replace the vmlinuz and initrd.img on an ISO9660 filesystem.