The issue with testing companies is that they are paid, and therefore do what you ask.
I do not think this is an issue at all! There is no way you could persuade a licensed professional to do the test the wrong way, or produce fake results, or skewed results. I am sure he would insist on installing the correct instruments according to code, and I said, the codes are detailed and admit no creativity. You put a dial thermometer 6 feet away from the boiler on a level pipe of a certain diameter made of a specified grade of steel, etc., etc. As I said, if the authorities found out the guy did it the wrong way, they would revoke his license. I am serious. This would be like asking an auto mechanic to install equipment that is unsafe and violates EPA regulations, such as a "rolling coal" accessory:
If a mechanic installs one of these stupid things and you cause an accident with it, he will lose his livelihood. No sensible mechanic would run that risk. It is like working for a chop shop (a place where they take apart stolen vehicles to sell the parts).
The only stumbling block with the Rossi gadget might be that if a licensed boiler technician measured the output and found it was over-unity, say 200% of input, he might not sign off on the report. He might be afraid to, because it would hurt his reputation to be associated with the claim. However, you could look at his results and draw your own conclusions. You could decide the next steps from that.
(On the plus side, you wouldn't have to pay him if he refused to sign off!)
To take a real-world example, as I recall, the Dean of Mech. Engineering at Georgia Tech refused to sign off on the results from Thermodyanics. They paid him tens of thousands for the test rig, but he and another professional reneged and never signed off. I doubt that he expected it would show a positive result.