I had to pull strings to get this done at the CP in Springfield, IL for a convention I was attending.

The hotel said NO, PC said NO adn then a manager finally said YES. I had to pay $139 instead of the $109 convention rate, but I was in the hotel I wanted to be in.

Plus, they put me in a suite, it was sweet!

Go above the regular platinum desk person's head to a supervisor, and make sure you read to them from their own website:

Guaranteed Room Availability for Platinum Members: When contacting the Priority ClubŪ Service Center directly and guaranteeing the reservation with a valid credit card, each Platinum level member will be guaranteed one room for personal use, for reservations made at least 72 hours prior to the date of arrival, except during special events that result in extraordinary room demand, as determined solely by the hotel. The member will not be charged more than the prevailing rate for the accommodation requested, and the member is not entitled to a rate less than the prevailing rate. Once made, Platinum Guarantees may not be dishonored for any reason. If the member does not arrive and has not obtained a cancellation number prior to 6 p.m. (4 p.m. in Europe), the credit card may be billed for one night's room and tax. Platinum Guarantees do not apply to Reward Nights.

Do note the caveat they have in there: except during special events that result in extraordinary room demand, as determined solely by the hotel.