Never seen this documented anywhere, but I just found out that you can create a syslinux boot disk from FreeBSD:-
This doesn't do anything at the moment apart from booting up syslinux. It is left as an exercise for the user to come up with a useful syslinux.cfg
Just make sure that da0 is a device is you can afford to mess with.
Code:
pkg install -y syslinux
gpart destroy -F da0
gpart create -s mbr da0
gpart add -t fat32 -s 4G da0
gpart set -a active -i 1 da0
newfs_msdos -F32 /dev/da0s1
cp /usr/local/share/syslinux/bios/mbr/mbr.bin /tmp/mbr-syslinux
dd if=/dev/zero of=/tmp/mbr-syslinux seek=440 bs=1 count=72
gpart bootcode -b /tmp/mbr-syslinux da0
syslinux --install -f /dev/da0s1
This doesn't do anything at the moment apart from booting up syslinux. It is left as an exercise for the user to come up with a useful syslinux.cfg

Just make sure that da0 is a device is you can afford to mess with.