...I was sort of hoping that HTP did something a little more useful than just exposing one stop to the left...
It does, if you are shooting JPEG: it adds a different tone curve to open up the mid-tones and shadows. Not that you could not come up with the same (or better) curve yourself though. In RAW it does nothing, except underexposing one stop.
... as long as you dont blow out any of the channels, exposing to the right would make tonal gradations smoother, not exposing to the left...
Correct, except for the "as long as you don't blow out any of the channels..." part. Well, how do you make sure you are not blowing out any of the channels? Only by underexposing, and that is exactly what HTP does.
... And why is iso 100 not available if all that is happening is simply 1 stop underexposure?...
Well, because you are already at iso 100, i.e., that is how the underexposure is achieved. In other words, HTP does not change shutter speed/f-stop combination, it simply uses one step lower iso, so if your starting point was iso 200 (the minimum for HTP), it actually shoots using iso 100.