The Nano variant uses md(4) rather than tmpfs as a ram drive. You could use that.
[2.2-RC][root@xtm5.localdomain]/root: mount -p
/dev/ufs/pfsense0 / ufs ro,sync,noatime 1 1
devfs /dev devfs rw 0 0
/dev/ufs/cf /cf ufs ro,sync,noatime 1 1
/dev/md0 /tmp ufs rw 2 2
/dev/md1 /var ufs rw 2 2
devfs /var/dhcpd/dev devfs rw 0 0
You can move /tmp onto a ram disk on a full install using the options in System: Advanced: Miscellaneous:
Edit: Although..https://forums.freebsd.org/threads/tmpfs-dev-shm-md-questions.45210/#post-256239
Steve