And supposing that it can't be shown that nothing is intrinsically rational, then perhaps meta-ethics is fairly trivial and cannot entail moral nihilism.
Moral Nihilism = Nothing is morally wrong. Moral nihilism here is not about what is semantically or metaphysically possible. It is just a substantive, negative, existential claim that there does not exist anything that is morally wrong.
i.e. there is something that is moral. This is entirely trivial, and how could anyone object: except that this "morality" makes no sense?
So to assuage any interest in morality, I think moral discourse, whatever its status as true or false, real or unreal, subjective, objective, relative or absolute (though the impossibility of either of those would have bearing), must also be possible.
Non-cognitivism famously claims that we don't assign properties in moral judgment. But does anyone say that moral discourse is impossible?