You'd need to edit the boot script (/sys/lib/sysconfig/fl/boot if I remember well)
to setup your fs dev before starting fossil. Take a look at the bootdir section
of the config file, where your boot script is mentioned. We use something like this:

#!/boot/rc -m /boot/rcmain
cpuserver=yes
...
echo mirror once /dev/sdC0/once /dev/sdD0/once >/dev/fs/ctl
echo mirror fossil /dev/sdC0/fossil /dev/sdD0/fossil >/dev/fs/ctl
echo mirror other /dev/sdC0/other /dev/sdD0/other >/dev/fs/ctl
echo mirror other1 /dev/sdD0/other1 /dev/sdC0/other1 >/dev/fs/ctl
...
venti -w -c /env/venticonf -a tcp!127.1!17034  &
sleep 10
venti=tcp!127.1!17034
fossil -c '. /env/fossilconf'


Where both fossilconf and venticonf refer to /dev/fs/{once,fossil,other,other1}.