Hex editing in Modern Warfare is only used so far for the offline profiles. And even so, there is no packet being sent by your profile, its being sent by the amount of ammo you have that is determined by the gun. This sends ONLINE packets to show that you used ammo on (whoever). If you are going to have any luck, figure out how to enable a code of hex string that can be enabled during game.
Thats the only way I can see it working.
Sorry.