It's an interesting conundrum, to be sure. As I mentioned in my original post, we get into splitting hairs about what one would know from playing the game vs. what one would know from extensive testing. It's one thing to know that a particular craft you've built in other career saves will be able to do x, but entirely different to build and rebuild and rebuild in several different ways to find out how to push the limit of what it can do just so you can complete a challenge and earn that coveted badge. Where does the line get drawn? What separates the two? If someone knows, then they are smarter than I am.
To your question, I'm not sure there is a real solution here, other than "Don't Ask/Don't Tell". I mean, this whole thing wouldn't have come up if the original poster hadn't explicitly come out and stated what they were doing. Had he not said anything, nobody would have asked if he was testing outside the Caveman career he had going on for the NCD difficulty. But, if you are looking for an honest, let's-try-this-and-see-what-happens solution...
Perhaps make it a rule that, in order to try any difficulty harder than Normal, you must complete the easier difficulties in order? For example, if you want to do Moderate, you have to do Normal first. You want to do NCD, then you have to complete Apatite, Vandadium, Topaz, Corundum, and Diamond - in that order - before attempting and submitting NCD? I don't know if it's a good idea, but it's the one I have.