This sounds like it'll be a huge contribution to Manifold, especially when paired with arbitrage bots that keep the market prices on Manifold in line with those on real-money platforms or any other platform that anyone might think of as a more authoritative source of predictions.
My thinking is that you start with arb bots that force the market prices on Manifold to match the mirrored markets. If those bots make mana, great! Free mana. As soon as those bots start losing mana, that's even better. You can pull the plug on the bots and also shout it from the rooftops that Manifold's market probabilities are provably more accurate than where the markets are mirrored from.