What would it take to add additional slots on the body, such as a pants slot, a shirt slot (for under the armor), and additional ring slots (one for each finger)? I assume it's either very difficult or can't be done, since otherwise I'm sure someone would've done it by now...

It can't be done right now. There is some possibility that this may be an option in the future, but it depends entirely on what DragoonWraith does with NiSE.