I would like to write an essay about that eventually, but I figured persuading PUs of the merits of HU was lower hanging fruit.

For what it's worth, I have a lot of sympathy with your scepticism - I would rather (and believe it possible to) build a system resembling ethics up without reference to normativity, 'oughts', or any of their associated baggage. I think the trick will be to properly understand the overlap of ethics and epistemology, both of which are subject to similar questions (how do we non question-beggingly 'ground' 'factual' questions?), but the former of whose questions people disproportionately emphasise.

[ETA] It's also hard to pin down what the null hypothesis would be. Calling it 'nihilism' of any kind is just defining the problem away. For eg, if you just decide you want to do something nice for your friend - in the sense of something beneficial for her, rather than just picking an act that will give you warm fuzzies - then your presumption of what category of things would be 'nice for her' implicitly judges how to group states of the world. If you also feel like some things you might do would be nicer for her than others, then you're judging how to order states of the world.

This already has the makings of a 'moral system', even though there's not a 'thou shalt' in sight. If you further think that how she'll react to whatever you do for her can corroborate/refute your judgement of what things are nice(r than others) for her, your system seems to have, if not a 'realist' element, at least a non purely antirealist/subjectivist one. It's not utilitarianism (yet), but it seems to be heading in that sort of direction.