Back in my college days, I was a tuba player in the University of Washington Marching Band. I don't know for sure if he was the absolute first, but if our band director - Bill Bissell (better known as "Mr. B") wasn't the first, he was certainly an early adopter of using a massive US Flag during the pre-game playing of the national anthem.
For a couple seasons, the huge flag we used was wool, and we weren't allowed to get it wet (and in Seattle, game day rain wasn't exactly uncommon).
So we practiced two pre-game shows - one with the huge US flag, one without. Then on morning of the game the decision was made if we'd use the flag depending on the weather. Eventually Mr. B convinced an alumni group to finance a new nylon flag that we could use regardless of the weather. In the 1982 Rose Bowl, the UW was the home team and we did the pre-game show with that massive American flag - it was beautiful.
Mr. B passed away about 15 years ago, but now every time I watch a pre-game show with a massive US flag I smile and thing fondly of Mr. B.
I like to think that somewhere Mr. B is smiling too.