This is a question more than a comment, but wouldn't it be easier to avoid the whole testing procedure and just attack on the "not for street use" angle?
Some jurisdictions in the USA (notably California) are trying to go down that route.
Problem #1: I (hypothetically - I'm not a very good welder, but just sayin') can take any EPA or e-marked muffler you care to name, cut it open, rip the guts out of it, carefully TIG weld it back together, grind smooth and re-plate, so that it keeps ALL of the EPA labels or e-markings in place, but is completely obnoxious and loud. If one cares to do so, one could even do this in a way that it still passes the "broomstick test" (broomstick handle won't pass all the way through) but it's still obnoxiously loud.
Problem #2: Some aftermarket exhaust systems are quieter than some original-equipment EPA-marked or e-marked mufflers.
Problem #3: Requiring only EPA-marked or e-marked exhaust systems screws over the owners of vintage bikes, which (A) may not have had EPA markings or e-markings in the first place even when they were originally built, and/or (B) no longer have replacement original-equipment mufflers available AT ALL from the original manufacturer, or the parts are available but at extremely high cost.
Problem #4: Requiring EPA markings (as California is doing) screws over travelers through California who own foreign-registered bikes. EPA is US-specific. Bikes not originally sold in USA don't necessarily have EPA markings. For example, a bone stock Honda CBR125R does not have an EPA-marked muffler. (It's e-marked because it's sold in Europe, and Canada recognizes that particular European standard, but the USA doesn't.) Technically, owners of such bikes ought to be protected by the internationally recognized Geneva convention on road traffic ... but the USA never really implemented that properly, and this is why US (and Canadian) road signs and certain traffic laws differ from those used EVERYwhere else in the world. The theoretical possibility exists that an owner of such a bike in California could get in hot water in California after January next year for this. (Unlikely to ever happen, but still, the theoretical possibility exists.)
It's better to have an objective test. The only trouble I see with SAE J2825, is the method by which engine RPM is established.