All users have same Avatar from script

[Bilderback]

I have written a php script which pulls the avatar from the XBox Live Gamercard and want it to display as the default member avatar.
I have disabled avatars in the forum and wish to display one url for all members.
example:
/avatar.php?tag=$userinfo[field5]
The above line gets the members avatar dependant on their Gamertag (field5)
else it shows the default XBox Live question mark image.
How can I edit vbulletin to display the $avatar as the url?
I am willing to edit core files if I have to...
I have tried default avatar plugins and they do work, but avatars have to be enabled which in turn, allows members to change the avatar.
Being an XBox related forum, I would like to set the avatar with no user option to change.