Ok so have no idea if this has been posted or not but i just want to help anyone that used to do this on mw2 all the time (like me) and wants to also do it on black ops.

First off u must have already made ur moodded gt using ezgt and loaded it to ur harddrive, etc....this can be found on google

so now, with the modded acct of ur choice already on ur harddrive, follow these steps.

1 load BO without signing in to any acct
2 go to multiplayer, local, system link and it will ask u to sign in.
3 sign in to the desired modded gt NOT
4 start the match then QUICKLY sign out of the modded acct and into ur real xbl acct
5 if done correctly it will say UNABLE TO START MATCH etc...note: if done wrong, it will say your acct has been signed out. If this happens just restart the process
6 now go to multiplayer and it will be the same rank etc. as ur xbl acct but say the modded acct name!!!!

hope this helped anyone that was confused.
Sorry for my terrible grammar/spelling/abreviations

THANKS and +REP PLEASEE!!!!!!!