This answers your own question. If there are explicit rules that everyone agrees to abide by, by virtue of participating in the registry, then it is a rules violation to list a cracked-out coin as being certified. (A coin must remain in the NGC holder to still be "certified" ,with the exception of some Ancients, which are photo-certified by NGC.)
I don't agree. The first argument is (as I queried orginally): What to the rules explicitly state? If the rules set an explicit form of conduct (i.e. "certified coins only"), then that should prevail. If there are no such explicit rules, then the second criteria would be "will actual harm occur". But as you just explained, there ARE rules about coins having to be certified only, so given that, I would agree with you that there is an ethical violation in the OP scenario.
Regarding the infamous Norweb Hibernia, in order to turn it into an ethical dilemma, I was required to discover whether or not the PCGS guarantee explicitly covered a stated provenance for a coin, and as it turned out, it did. Therefore, that as well was a violation of a stated rule.
Great discussion, Coinbuf!