What if your scheme was implemented and as a result the hosted mining schemes decided to publish the ROI as a way of measuring their honesty. So each week/month they would say that they were paying out a certain percentage according to how much a client had invested.
 This would easily be verifiable by the investors by looking at what payout they receive.
 Over time we could see who had better returns and a result people would end up moving from the dishonest hosted mining schemes that weren't declaring mined blocks and investing to the schemes that had the consistently better returns.

 Mining pools would fail because the miners would cheat the pool so everybody would end up investing in hosted mining and specifically in the top performing hosted miner who would eventually control all of the mining.