You should create a blockchain of dashwallet connections. Each connected dash wallet to a specific masternode shoud be broadcasted to all the rest MNs. All MNs will sign this connection as being a real one, and this will create a "bandwidh" blockchain. So that the more dash wallets a Mastenode serves, the more are the masternode's earnings. The important thing is who verifies the captcha whenever the wallet connects. So the captcha verification should take place to an independant and randomly (using cryptographic randomness) selected masternode , and of course not to the one that the dashwallet initialy connects due to the high bandwidth. And of course captcha service , should also be paid along with the bandwidth service provided to the wallet. How much paid? i Dont know, you should vote using numbers.