We used our Platinum perk for dinner on our recent Dream cruise, but I have seen no recent reports of anyone unable to use it for brunch. I am guessing that the computer system in Palo automatically deducts $45 per person in a Platinum room (or the cost of the dinner, if it's less than $45) without regard to time of day. We spent less than $45 per person for dinner but did order one alcoholic drink. Although we didn't say anything to our server about the Platinum perk when we presented our KTTW card at the end of the meal, when he returned our receipt he apologized that we couldn't get the full $45 -- we paid only for the drink (and gratuity).
If you are concerned about brunch being covered, you could stop by the area for Dining Changes (probably outside Enchanted Garden) on embarkation day and ask the Palo representative.