Welcome to the boards! I am a huge fan of the marching arts. Years ago, I was in my high school band, as well as competing in a Drum and Bugle Corps. I played a few different instruments there, and was in an indoor winter colorguard as well. Anyway, all of that is to say that I can appreciate wanting to get some practice time in whenever you can.
First, I would say that there may not be a specific prohibition on your equipment, but they may not like you tossing a 5 foot pole around. Disney would be liable for any injuries, and while we all know that you would be in complete control, they might not want to take the chance. Secondly, should you not run into any issues with the resort allowing you to practice, you almost certainly will have an audience. There are absolutely no secluded places around WDW, and something like what you're asking is definitely interesting, and people will likely stand around to watch, or even stream or take pictures, which may make you uncomfortable.
My advice would be to leave the flag at home. I know you'll be missing out on valuable practice time, but the time might not be as useful as you think, especially if your head isn't into it because 1: you're on vacation in WDW, and 2: half a dozen people are staring at you (though that can help with any stage fright you may have come gameday). Enjoy your time on vacation, and hit the flag work as soon as you get back home.