I had this setup in the past. In the bootloader, besides the regular options, I had the option to boot XP instead. Nice and clean dual booting without installing extra bootloaders or anything. It was the equivalent of escaping to the loader prompt and issuing
chain disk0s1:
. I remember all I had to do was adding a line to /boot/loader.conf, something like "chain=disk0s1", and the option would appear in the bootloader and simply work. Problem is, I can no longer find what the line was supposed to be. I've went through loader.conf's man page but simply couldn't find it. Anyone remember how to set it up?