If this is a non-sanctioned club match then the local club can handle it pretty much as they wish.
If this is a sanctioned or "official" IDPA match then the shooter is warned about illegal equipment or equipment placement. If they persist after warning it is a Failure to do Right or FTDR and another warning. If they still persist, or in this case if the appendix carry position causes them to sweep themselves or others during the draw, then it is a match DQ and probably an invitation to not come back.
There is usually a good reason for rules but even if the reason is just "that's the rule" you always have the option of either following the rules of not playing the game. Heck, you can even do what Bill Wilson did and go invent your own game. You might start the next "big thing"!