See if you can remove the nipple before you bother contacting anybody about a new nipple. Unless my 3M clone (currently in front of me) is dramatically different from the Milwaukee, there are no flat surfaces to use a wrench on, and it's not even clear that the nipple is attached via threads of any sort (size or standard), aside from a hairline crack between nipple and regulator body.
In short, I believe it's a one-piece unit, and there's no possibility of replacing the nipple. But I could be wrong.
In any case, even if it's possible to replace the nipple, getting Milwaukee to send you a couple of proprietary washers is still the cheapest option.