Any globally unique signature in Ardupilot hardware, or Arduino in general?

I have several APM 2.5 boards and need to identify them based on some globally unique hardware signature that does not change with programming.

Arduinos and atmel AVR chips in general do not have (also this thread) an accessible serial number.

However, it seems that the Ardupilot has so many integrated sensors and other ICs that one of them must have something unique I can use ( see schematic )!

I will be checking datasheets for MPU-6000, HMC5883L-TR and MS5611, but in the meantime, if someone has already figured this one out, please answer.

me@mycompy:~/arducopter/MissionPlanner\$ dmesg|tail