solved via a tip from Paul, with the addition of the following lines
In addition, the guide I was using mentions defining md filesystems in
md /var mfs rw,-s8m 2 0
md /tmp mfs rw,-s8m 2 0
...instead of in rc.conf, as:
This method (fstab) proved not to work:
mount: /var/tmp: No such file or directory
Mounting /etc/fstab filesystems failed, startup aborted
I'm not sure why, but I'm happy it's now working.
Thank you all for you input.