I don't see why not. Coastdown tests could check smokehouse's claim, with decent methodology. Or a team could observe a lot of amateur races, noting everyone's choice of wheels and you could get some information with enough data points. You could monitor power output against speeds on training rides of elite riders, given different wheelsets. You could measure the various mechanical responses in a lab. I think you'd want blind testing only for subjective evaluations - but that's easy isn't it? Just don't let them see the wheels before getting on the bike.

Now this part is just my opinion: I expect that you're right that we won't see unbiased, analytical comparisons but not due to the technical difficulty of testing. Because it would be expensive, and I suspect that they'd show less difference than is commonly assumed, hence counter-productive for marketing. Without good marketing reasons there is no motivation to spend what it would take.

I'm still trying to figure out if even, in all situations, a heavier wheelset is necessarily detrimental in hilly routes. Or whether flexier wheels really rob you of power, or impact handling other than in extreme cornering. I know what people say, but people say a lot of things that don't necessarily hold up and that's particularly true of the things they've spent a lot of money for. Like you, I don't hold out much hope for objective testing though.