Pretty sure if you go back and read the rule book, regardless of whether you think or assume who was and was not running nitrous in pro stock when The Dodge Boys were accused, that the rule book was quite grey when it came to the fuel in Pro Stock. Just like some of the Nascar guys that used to put fuel in the frame rails, the rules limited the size of the fuel cell but didnt say you couldnt run huge fuel line or store it in other places.

I would definitely agree that an optical sensor to pick up the top bulb would be a very easy place to cheat.