I would also like to know the answer to this, but was wondering if you can still do this if you have booked a package holiday through a tour operator and not booked the hotel direct.
It depends who you booked with. If you booked directly with Walt Disney World, give them a call and they will add the request to your room reservation. This is the BEST way of requesting a room - the majority of requests made in this way are fulfilled.
If you did not book direct with Disney, you can send a fax to your resort (addressed to the room assigner) with the details of your request. However, lately rumour has been spreading that Disney are getting fed up with requests made in this way and are often not fulfilling them. The only other option is to wait one or two days before you depart and see if CRO (Central Reservations Orlando) has your reservation uploaded on to their system.