Obtaining UL certification (where I presume UL is 'Underwriters Laboratory) is in contrast, I have been told, an expensive and highly formalised process
Yes, it is Underwriters Laboratory, and yes it is formalized and it costs a fortune.
UL is a corporate entity, with no governmental authority, but it has de facto authority. It is an insurance company consortium. No insurance company will issue a policy to ensure the safety of equipment not certified by UL. No store or equipment supplier in the U.S. would think of selling a machine not certified by UL. If something went wrong and there was an injury, death, or property damage, the store would be sued to a fare-thee-well.
I doubt UL would work with Rossi as an individual. He would have to have a company, a factory and employees. See:
If another company had a contract to manufacture his devices, they could have the product certified. They would have to provide UL with every single detail about the product, including the vendors of the faceplate screws, and with several working prototypes the same in every detail with the final commercial product. Needless to say, there is no such company or prototype.
I am pretty sure that in course of doing this, UL would measure the efficiency. They do with things like air conditioners and boilers. Assuming the thing works, they would find it is "over unity," meaning it produces energy out of nowhere, or with unknown nuclear reactions. That would bring the approval process to a halt. No corporate regulator would consider approving a machine that works by unknown principles. That would be insane. Rossi would first have to wait for the National Laboratories and the academic establishment to figure out how and why the thing works. That would take years and cost billions of dollars. I am sure it would be done, and it would be worth billions of dollars, but anyway, I am sure you cannot sell nuclear reactors that work by unknown principles anywhere in the world.
Assuming the thing does not work -- which I am sure is the case -- UL would not approve it because it does not work. Unlike the Patent Office, they are in the business of confirming that equipment does what it is claimed to do in the specifications.