You probably will get more response by asking on the freebsd-stable mailinglist or on freebsd-hackers. Especially if it used to work on FreeBSD 4.x and 5.xx
I wonder if there could be a sysctl variable you need to turn on. Unfortunately don't have a FreeBSD box available at this time.
OpenBSD has a sysctl to enable directed broadcasts, which is not what you want
Code:
$ sysctl -a | grep broad
net.inet.ip.directed-broadcast=0
Only after setting this to 1, a directed broadcast would work under OpenBSD.