From the Marriott Platinum Elite Benefits page (http://www.marriott.com/marriott-rew...ts/platinum.mi):

"Room Upgrade: For Platinum Elite members the best way is up! Based on room availability at check-in and limited to a Member's personal guestroom, we’ll do our best to upgrade your room."

Asking a hotel to upgrade you prior to check-in is essentially asking for special treatment that is not part of the Marriott Rewards program. While some hotels might comply, others will follow the rules verbatim.