I had actually thought about that this morning as well... Damn, not even one spare pin :) Do you need all of the FTDI pins?
Looking at the schematic, I believe you are correct. A piezo element could be driven from Digital 0 or Digital 1. And since piezo elements have a relatively high impedance, I'll bet the watch would still be programmable via FTDI, although reprogramming would likely make quite a racket!