I am not sure that all "tests" conducted by shooting magazines are "significant" by any stretch of the engineering or statistical imagination. Never really stopped anyone from doing them.
I am kind of surprised no one that does action trueing hasn't already done it and published the results to brag on how much better they shoot after a trueing.
It would be easy to do, just use a Criterion nut barrel. In fact you could use SEVERAL barrels and calibers on the same action. Just to make it interesting.
Well, if a gunsmith did the test, do you suppose he'd admit that he couldn't detect any difference?.................no, I didn't think so.
But there is a broader difficulty in testing action truing. Contrary to what you say, this test would NOT be easy. Or more correctly, while the test might be easy, finding proof would definitely not be easy. Here's why.
Most of us test by shooting paper targets at ranges between 100 and 1000 yards using bullets shot out of a rifle held by a human in conditions where the bullet travels through an unknown atmosphere. The loads, although we try to make them as uniform as possible, are subject to all sorts of variations caused by case volume differences, primer inconsistency, bullet diameter, weight, and shape differences, barrel temperature, etc. etc. etc. Otherwise, our bullets would hit the target where we aim each time we pull the trigger.
When you test a before-and-after action, I would imagine you're looking for improvements much smaller than those you might experience by differences in barrel temperature, to use one example. In fact, of all the things that come to mind which make our hand loads less than perfect by the time the bullets get to the target, I would guess that action truing is way down the list of significant factors.
In other words, teasing meaningful data out of a data set which is chock full of noise when you expect only a tiny difference in the particular factor you're testing is fraught with uncertainty.
But that doesn't mean it's not worth truing your action just because you can't prove that it helps accuracy. It might be well worth the cost just to have an action which cycles better and feels nice. I believe that any accuracy "improvement", if it actually exists, is just like many of the tiny steps we take with our hand-loads and equipment preparation in a determined attempt to get a leg up on our fellow match competitors. Can you prove how much normalizing your primer pockets helps your group size or SD figures? Can you prove that annealing every time is the best routine. I can't and neither could Brian Litz when he examined annealing frequency...... but I do it anyway and I keep my fingers crossed that dotting all the I's and crossing the T's will add up to an advantage. If nothing else, it gives me increased confidence, which is more important than some people might imagine.