You should be able to get the Customer Service address from the web site. If not, you could certainly get the address at a NW ticket counter.

If you write a complimentary letter to this address it should end up at the proper department. I recently sent a letter of commendation to CO and within a week had a letter from Gordon Bethune's office thanking me and telling me that they would personally thank the gate agent and put my letter in his file. I would hope that NW would do the same thing.

P.S.
I saw the same agent this past Saturday. I told him about my letter. He said that yes, he had been thanked and he then thanked me for taking the time to write.