If you call the Office and are polite and nice, frequently the receptionist will tell you who your reader is--not always but frequently. The Admissions Office is very understanding about the plight of WL kids and try as best they can (but remember they are dealing with trying to woo the kids they admitted right now as well as distraught and frequently very angry rejected candidates/families/school counsellors so, please cut them some slack--it will go a long way for you) to be helpful.
Writing your regional reader if you can (if not a name, then in the email, note where you are from and ask the Office to forward the email to the appropriate person...) is very important as it will be her or him that has to go to bat for you if you have any chance of making it off the WL. The committee meets whenever they have WL decisions to make and they start afresh looking and analyzing applications. If a reader REALLY likes a candidate it can make a big difference as there will be more great kids (all of you who made the WL are great kids) than slots so having someone passionate about your admission is critical.
The fine line is how much to contact -- you want to keep you name fresh in the reader's mind but you don't want to come across as a creep-oid who the reader rolls her or his eyes when she, he sees your email in the inbox. (That is the quickest way to doom of all....well, probably not as bad as the kid who handcuffed himself to the office front doors--they probably have an APB to all Harvard professional and graduate schools about him