This situation implies that the kid shouldn't be marching in the drill at all, as they're a hazard in some way to the kids around them. Making them an "alternate" then begs the question of whether they get to march at UIL when two kids fail out of the line and you only have one other kid to plug the hole. Do you want to have the conversation with the parent justifying why their kid who has worked as hard as they can doesn't get to march even though there's a spot for them? Do you want to have that conversation with your admin and your parent's advocates and attorneys?
OP: Just find a way to include the student with stationary drill. Judges don't know and don't care what your program circumstances are or what a kid's IEP says. Do what's best for your band as a whole.