Address the letter to the admissions committee...it will most likely go straight to your file where it will be reviewed when your file comes back up for review.
I just sent a letter to ECU (I'm not waitlisted yet...they reviewed me once and will review me again). I told them all about what I've done since I interviewed with them 2 months ago. For me this included presenting 2 posters and co-authoring an article at a national convention and being taking on new leadership roles within my EMS squad. Obviously, much happened in the past 2 months and I won't have another big update like this again!
If they still haven't made a decision on my by Christmas, I'll most likely send them another letter briefly describing my continued involvement in all my activities and expressing my continued interest in the school.
I hope this works. Please ECU, please oh please oh please!!