Koshi is the back of the hips, translating as the lumbar pelvic centre. Just near the base of the spine.
The basic set of definitions in Nelsons (admittedly, not a specialized dictionary for Chinese medical usages) has the following:
Nelson 3799: koshi hips, loins, waist, pelvic region; small of the back; haunch; lower-panel wainscoating (lower wall only)
Afterwards come the compounds. So, a couple of questions:
First, do you have a citation in which it is quite clear that Tohei (or any other Japanese martial artist) is using koshi in a way that refers specifically to "small of the back" and not the entire region, which includes "hara?"
Second, while all these points or regions play a role in direction and alignment, how could a ground force "originate" or be "centered" someplace other than the ground? Wouldn't that make it some other kind of force if it were?