This pic (ignore that is says for a 50° vanagon - the serp bits are the same) shows why the A/C and waterpump cannot both be run by the serp belt at the same time without spinning the water pump backwards (or adding a custom idler). With the belt going around the A/C compressor pulley, there is no engagement with the water pump pulley.
Weird. This time I wasn't the one who deleted my post...