To the people wondering "Hey, how do marching bands handle non-even time signitures?", the answer is, mostly they don't. Most stuff I've ever played, or heard played, is generally metrically simple.
After that, how it is handled is up to the choreographer, of course. The easiest solution, if you insist on playing something funky, is to be standing still at the time. But beyond that, you really can do anything you want with training and practice, it's just that you don't have an infinite amount of training and practice time. Nothing stops you from marching around to a 7/4 song, except that you'd have a lot of learning and training and practicing and possibly the odd collision resulting from fighting years of habit...
... and ultimately, the crowd probably wouldn't care in the slightest, so why bother? Crowds like sticks being tossed around and fancy maneuvers and dance steps and a bit of shouting and cheerleaders (if at all possible), not musical complications like non-standard harmonies and metric complications.
(Edit: Someone will probably make some sort of comment about "collision", and I'm about to go to bed local time, so: My personal best is trombone-on-trombone, full-on bell contact with a full-speed 180 contact... while wearing braces. I'm not sure how you can beat that, for sheer concentrated impact with a band instrument. I was out of commission for two days, which, all things considered, probably wasn't that bad. Braces + trombone was already a bit masochistic...)