There is no P2P communication in WoW, only client-to-server one. Check your firewall (such as Outpost), it will show that even in capital cities WoW opens only one connection.
Direct communication between clients is a source of many problems, one of them is cheating. GunZ the Duel (MMO third-person shooter/fighting) uses this concept and it's not too good for players.
Also, TCP protocol won't allow you to block one packet and pass another. TCP is aimed for data flow integrity, meaning the loss of one packet auctomatically disconnects you.
Yeh, and WoW protocol is encrypted :P
Thanks for good laugh and sorry for ruining the hoax ^^