enabling directed broadcasts you enable your network to the smurf attack!
There is an other trick:
add a static arp with a defined ip for each interface, pointing to the mac ffff.ffff.ffff
eg.
arp 172.16.10.254 ffff.ffff.ffff ARPA
arp 172.16.11.254 ffff.ffff.ffff ARPA
arp 172.16.12.254 ffff.ffff.ffff ARPA
Then you generate the magic packet with a destination 172.16.12.254 and subnet 255.255.255.255
The packet will be delivered to all hosts on the subnet 172.16.12.0/24 and will wake up the PC with the MAC defined in the magic packet.
This solution is NOT subject on the smurf attack because nobody will have the "virtual" IP address and nobody will answer to them!