Silly answer:
Pau = 1.5 * pi is the way to go. It'll please nobody, so nobody will feel left out.
Proper answer:
Notice how next to nothing is abbreviated with the letter pi. Since pi has been the traditional constant used, everything else more or less decided not to use the letter pi. Had tau been the "one true constant", I'd expect the opposite to happen - the letter tau practically reserved for 2pi and the letter pi more or less free to use in most contexts.