mmm... assuming your EEPROM is blank at it's first power-up. You may use a spare i/o and dump a Serial number in? If there's no SerialNumber in the EEPROM, the whole thing just don't work... Still possible to enter it manually when you burn your PIC. All my PIC Programmer software allow that.

If your PIC have it and your Device Programmer allow automatic ID serialisation, you could still use the PIC ID?