Although I've read the metaethics sequence, that was a long time ago, but I think I'll put reading it again on my todo list then!

My intuition behind thinking 1 unlikely (yes, it's just an intuition) comes from the fact that we are already bad at generalizing "people-ness"... (see animal rights for example: huge, unsolved debates over morality, combined with the fact that we just care more about human-looking, cute things than non-cute ones... which seems to be pretty arbitary to me). And things will get worse when we end up with entities that consist of non-integer numbers of non-boolean peopleness (different versions or instances of uploads, for example).

Another feeling: also in math, it might be possible to generalize things, but not necessarily, and not always uniquely (integers to real numbers seems to work rather seamlessly, but then if you try to generalize n-th differentials over real numbers... what I've heard, there are a few different formulations that kind of work, but neither of them is the "real one".)