i selected /dev/da6s1 as the zfs root partition. unfortunately, during grub2
installation, the installer tries to setup grub in /dev/da0 instead of
/dev/da6, which fails. instead of
grub-install --no-floppy --force "/dev/da0"
it should use
grub-install --no-floppy --force "/dev/da6"
There's a debconf template in grub-pc package to ask user which
disk(s) should GRUB be installed to. I think grub-installer should
either offer the same functionality, or just rely on this template
externally (if possible).
Please could you file a bug on grub-installer?