IMO, a device like this could be built for about $15, at a guess (not allowing for development or manufacturer's markup.) Clip one to each gun as it goes on the field, much like armbands on players, and the LED will light up if it senses two shots less than 65ms apart. If it's attached to the gun correctly, it only needs to be secure enough that it can't fall off - it's easy enough to ensure a good acoustic connection to the body of the marker - so a bit of velcro would work, and I'd be surprised if it needed any particular calibration - the vibration caused by a shot is well above any other kind of interference, whether it's a DM5 or a sewing-machine cocker. The trick is in the correct design of the unit and the mount.
The end of the barrel would allow for breakbeam operation (easier) but would be vulnerable to damage/accidental removal, especially for front players. It's got to be vibration-activated, and located somewhere that minimizes chance of being knocked off. I think know where that is, but I'm not telling yet
This is all just off the top of my head, but I really think it's doable, and cheaply. But of course the only thing it can enforce is ROF cap - it won't do anything for trigger modes (FA, illegal ramping, breakout modes etc.) and can't catch velocity ramping. Because of that, one has to ask whether it's worth it or whether we should be pursuing a more all-encompassing solution (just as soon as one comes along.)