can you react with flurry of feathers before you score your point for holding and win the game with the grand plaza battlefield?

No, you cannot react with Flurry of Feathers to help satisfy the win condition for The Grand Plaza.

According to RiftJudge FAQ #10151, The Grand Plaza's ability follows a strict "condition, condition, effect" structure. You must already have 7 or more units at that battlefield the moment you hold it during your Beginning Phase for the ability to trigger.

If you have 6 or fewer units at the moment you hold the battlefield, the condition is not met, the ability does not trigger, and you receive no priority window to play Flurry of Feathers to reach the threshold.

Furthermore, as noted in RiftJudge FAQ #10013 and #10068, even if the ability did trigger (for example, if you already had 7 units), you cannot play the Bird tokens from Flurry of Feathers onto The Grand Plaza if you are currently involved in a showdown or attacking that battlefield, as tokens must be played to your base or to a battlefield you already control.