Braces can be very hard on your mouth in general. I don't think OP will loosen anything, it's just food. It would have no more effect than if you swished water around your mouth. Actually I think it could remove some of that metal that may be leeching into your mouth, and that might be a good thing.
The thing you said about whitening around the braces, actually that was brought up a long time ago and the consensus is yes, you would most likely have a pattern. But if you continue to OP after they come off, then that should even out pretty well.
The whole thing is, braces or no, you should OP for your basic dental health anyway. Also it may take away some of the sensitivity your teeth might feel with the braces. You'll have to let us know...