Hmmm… That has an uncomfortable ring to it. I like to think of my co-competitors as mates, not as people who end up being part-time policemen.
What's wrong with raising it with the player directly instead? If I see someone at a tournament doing something that is against the rules, I'm not shy about pointing it out. And this need not necessarily be confrontational. (Quite often, a violation isn't intentional but due to inexperience, such as continuing multi-ball while a ball is stuck. In the heat of the moment, the player may not even notice.)
The real problem here is that it is near-enough impossible to prove sandbagging unless a player openly admits to it. If someone were to accuse me of sandbagging, my first question would be "how do you know I was sandbagging?" Any player can have one or two bad games out of the blue, no matter how skilled. How is a TD to know for sure that someone didn't just have a run of bad luck?
The root cause of sandbagging is money. The more money is at stake, the worse the problem gets. I see only two ways to change this: take away the money altogether, or get rid of the payouts in B, C, and D.
There are loads of other sports that struggle with this problem. Tennis is one of them. With international betting, organised crime gets involved, and it's downhill from there. An interesting report about this here: http://www.abc.net.au/4corners/stories/2016/02/01/4395832.htm
There are ways to statistically identify players who do not play to the best of their ability, by monitoring match outcomes and correlating that with suspicious betting activity. But I don't think this would work well with pinball.
What it boils down to is that people's morals and ethics differ. While there is an incentive for sandbagging, some people will engage in it, period. Ergo, the best way to stomp this out is to remove the incentive.