I know the local HRO will do it for you if you show them your MARS license. The conversation with a new trainee is pretty funny and usually goes like this:
Trainee: I just joined MARS, can you mod my radio for me?
HRO: Yes, but you need to show us your MARS license.
Trainee: I don't get a MARS license until I complete the training program.
HRO: Well then come back when you complete the training and have your license.
Trainee: Most of the training is "on-air", so how can i do the training if I can't get on the the air?
HRO: That's not our problem. Our policy is that you need to show us your license to mod the radio.
Trainee: What if I have the State Director or Training Assistant Director send you a letter stating that I'm a MARS Trainee?
HRO: We'll accept that instead of a license.
(later that day)
Trainee: Mr. Training Director, can you send a letter (etc.)
Training Director: Here's a link to worldwidedx.com where you can find the mod to just do it yourself or get a friend to do it for you if you can't.
Trainee: Wow, you can find anything on the internet these days...