Except for this one...which many seem to think is no big deal.
Rule 14-3
The United States Golf Association (USGA) reserves the right, at any time, to change the Rules relating to artificial devices, unusual equipment and the unusual use of equipment, and make or change the interpretations relating to these Rules.
If the belly putter stance/stroke is not an "unusual use of equipment" then I don't know what is...
Furthermore, the case can be made that the belly putter is in fact "unusual equipment"
There is plenty here for the USGA to work with it...no excuses!