I have heard (no first hand information ) that some resorts just throw the faxes in the garbage. I always call and ask to have my request added to my reservation.
I call 407-W-DISNEY. As far as I know, the phone number listed above goes to an off-site call center; you will not be speaking to someone at the Yacht Club. I'm not saying that they won't be able to add the information to your resie, I'm just saying that if you start asking specific questions about the resort when you call, they might not be able to provide accurate answers.