Even the head nod up is very regional and has to be read with the rest of the body language...That's more of a "what's up" while a nod down is "respect".
It's actually not that different here, I don't think. A nod down is very definitely respectful, while the nod up
can be in an informal, "what's up" sort of way, but as you say, it's body language and facial expression dependent, and can be much more of a social challenge as well.
Carl Sagan wrote a book of his speculations on human psychology, entitled
The Serpents of Eden. As a cosmologist, I think his knowledge of the subject is probably that of an interested layman, but his therefore largely unconventional ideas were pretty entertaining and thought provoking. I don't think he talked about the head-up nod, but I do remember a longish ramble about social groups in pack animals, lowering the head in submission and defense (as in dogs, wolves, primates, etc), to avoid a dominance battle, to maintain social peace, and the relationship with the variety of independently developed bows across the world, all of which seem to center around lowering the height of the head, generally the lower the higher the rank differential.
As I said, I don't
think he talked about head up nods, but from that view it's interesting to think of them as a bit of a reverse bow. If a bow is more or less related to demonstrating your respect for and in some respects submission to another individual, a head-up nod is somewhat of a denial that they are in any way above you. Which, I think, would explain its place as an extremely informal greeting among friends and equals, nearly a quiet, inoffensive assertion of dominance. In some ways making it more like a hand shake, which can certainly be an assertion of dominance for many, and a defense of position for many more. Which I personally like.
Not to say I don't bow in any situation where it is expected, and how is expected. Whatever a bow may signify in terms of respect, not to bow as called for when it's the norm is just plain disrespectful.