I don't think anyone has OPed and then had their braces off. But I do know some posters that do OP with braces.
Personally, I think that if there is some sort of mottled effect when they are taken off, you would just need to continue to OP after, and they will eventually even out. It will be like that anyway, and you should feel lucky you found a way to whiten teeth that is safe.
The one thing that comes to mind is how hard braces are on the dental health of the mouth. OPing would mitigate the damage done to the gums. I would most definitely OP while having braces just for that reason.