Arduino low power consumption and multiplexer
Single transistors don't really have any "quiescent current". If you don't feed them some current (or, in case of MOSFETs, voltage), they don't conduct. (A couple of nA leakage current can safely be ignored.)

As @geometrikal stated, these days there are better ICs for your application. Modern port expanders have an interrupt output, so you don't need the array of diodes which your solution would require. This particular IC has a standby current of just one µA, so you don't need to turn it off.