mount /dev/da0p1 /mnt

mount
/dev/ada0p2 on / (ufs, local, journaled soft-updates)
devfs on /dev (devfs, local, multilabel)
/dev/da0p1 on /mnt (ufs, local, soft-updates)

# cat /etc/fstab
# Device    Mountpoint  FStype  Options Dump    Pass#
/dev/ada0p2     /       ufs     rw        1        1
/dev/ada0p3    none     swap    sw        0        0


#User Added Entries
#/dev/da0p1      /mnt            ufs     rw,late  3       3

# PROVIDE: mountlate
# REQUIRE: DAEMON
# BEFORE: LOGIN
# KEYWORD: nojail

. /etc/rc.subr

name="mountlate"
start_cmd="mountlate_start"
stop_cmd=":"

mountlate_start()
{
        local err latefs
        sleep 5               <-------- Added this line
        # Mount "late" filesystems.
        #
        err=0

/dev/da0p1      /opt            ufs     rw,late 0       0

/dev/PARTITION.journal         /mnt         ufs     rw,async,late     3       3

iscsictl_enable="YES"
iscsictl_flags="-Aa -w 10"
iscsid_enable="YES"