If a school asks you not to contact them regarding the waitlist, I would respect that request. If you just want to make sure that the admissions office has received you letter, you can always send it certified mail with a tracking number, or you could include a self-addressed stamped envelope/postcard with your LOCI and a note asking the office to confirm receipt of the letter.
With that said, I think the circumstances may change later in the cycle. If you are placed on the waitlist in February, send your LOCI, and you do not hear anything by May, June etc... it may be in your best interested to again follow up with the school to let them know that you are still interested and to see whether they are even admitting anyone from the waitlist. Again, there are other actions you can take to show your interest. Just because a school does not want to be bombarded with emails and phone calls regarding the waitlist does not mean that you cannot show your interest by visiting the school, scheduling meetings with adcoms etc...