Perhaps I misunderstood. There are definitely possible scenarios in which metaethics could matter to a paperclip maximizer. It's just that answering "what meta-ethics would the best paperclip maximizer have?" isn't any easier than answering "what is the ideal metaethics?". Varying an agent's goal structure doesn't change the question.
That said, if you think humans are just like paperclip maximizers except they're trying to maximize something else than you're already 8/10ths of the way to moral anti-realism (Come! Take those last two steps the water is fine!).
Of course it's also the case that meta-ethics probably matters more to humans than paperclip maximizers: In particular metaethics matters for humans because of individual moral uncertainty, group and individual moral change, differences in between individual moralities, and the overall complexity of our values. There are probably similar possible issues for paperclip maximizers-- like how should they resolve uncertainty over what counts as a paperclip or deal with agents that are ignorant of the ultimate value of paperclips-- and thinking about them pumps my anti-realist intuitions.
Do you believe in an objective morality capable of being scientifically investigated (a la Sam Harris *or others*), or are you a moral nihilist/relativist? There seems to be some division on this point. I would have thought Less Wrong to be well in the former camp.
Edit: There seems to be some confusion - when I say "an objective morality capable of being scientifically investigated (a la Sam Harris *or others*)" - I do NOT mean something like a "one true, universal, metaphysical morality for all mind-designs" like the Socratic/Platonic Form of Good or any such nonsense. I just mean something in reality that's mind-independent - in the sense that it is hard-wired, e.g. by evolution, and thus independent/prior to any later knowledge or cognitive content - and thus can be investigated scientifically. It is a definite "is" from which we can make true "ought" statements relative to that "is". See drethelin's comment and my analysis of Clippy.