I'm assuming the op can view the hashrate of all the connected machines, which means that it should be theoretically possible for him to incorporate a way to prevent false advertising. A certain margin of error (say 5%) could be established as an acceptable range for the actual hashrate, if the hashrate falls below this, the payment to the rig operator could be prorated.  It could also be used to establish an official method for determining the stablity of a righ (kinda like a verified seller setup).