From my understanding, bow range is supposed to be horizontal distance, so do rangefinders adjust for stand altitude or is it just a point-to-point measurement?
You're correct in the assertion about horizontal distance, and it's what causes a lot of bowhunters to shoot high. Although there are a few rangefinders starting to hit the market that claim to be able to adjust for this, I haven't heard too many great things about them at this point.
You could shoot a few known objects while you were still on the ground, but I find it much simpler to just pick out a tree I want to range and follow its trunk up to the level of my treestand, which is usually 20 feet or so. I then range that tree at that horizontal distance, much as you suggested. That gives me the real distance I should shoot for.