The Platinum desk has nothing to do wth it.Other then to make your requests known to the hotel.
It is all based on the hotels in house decision to do so.

I will assume/guess depending on the inventory the hotel may have on hand they may vary procedures.This is high season to the best of my understanding so while my expectation is there I know also not to expect it for certain.
How I discovered in advance the past pre blocked upgrade was when the Plat desk emailed me my reservation at my request before my arrival.

Last I looked into my reservation I was still in a room.
Had not been upgraded yet. But the Plat desk has no control over pre blocking it is a hotel decision.The Westin Diplomat in Florida is another such property that has often pre blocked suites in the past.
Hope this helps.Best wishes on your hopeful upgrade .
If the past is anything to go on you should do just fine!