Uphill bound has the right of way may serve well as a general rule of thumb, but the specific location of the encounter matters a lot.
I vacation in CO frequently. I don't drive on overly technical roads, but some are narrow with rough patches. I was driving my 2WD truck down a mountain in a bit of a rough spot when I rounded a bend and encountered an uphill bound small SUV. I was aware of the rule of thumb but my truck being light on the back end couldn't get traction in the rutted loose gravel that I stopped in.I
I got out and told the other driver what was going on. They understood and offered to back up a few hundred feet to a place where the road was wider and they could pull over. Sometimes a friendly conversation helps.
If I am uphill bound and I encounter another vehicle, I don't mind backing into a passing location if I have one at my disposal. I find backing uphill in narrow, technical terrain to be more difficult than tackling the same terrain going forward.
It seems to me that the rule of thumb is fine, but there would be too many variables to codify it into law. I can say that empathy, patience, communication and cooperation go a long way.