I see where you’re coming from and IDPA has allowances for a “Specialty Division”, unfortunately holster position still has to comply with IDPA rules so it’s not really an IDPA match at that point. Whenever you’re dealing with a formal competition which has rules, semantics are important.
Here are the concealment rules right from the rule book. There’s nothing about your magazine carrier being concealed during the actual COF so theoretically at the beep there wouldn’t be a problem with sweeping back a cover garment behind mag carriers other than the fact it takes more time than its worth.
Your idea about using a towel or something that drops away would most likely run afoul of the fact it’s not a “garment”. In the same way I doubt it would fly if someone just stripped off their vest when the timer sounds.
Why do you think this would be advantageous enough to game so hard? I mean, I’m all about finding the absolute edge of the rules, I just don’t see where there’s any benefit to be had.