How to uniquely identify which PCB is plugged in / connected
You could use two resistors with a common to ground to give group and number within the group. 10 x 10 should be no problem with fractional-cent 1% resistors. I wouldn't bother with dividers- put the other resistor on the board doing the reading. 

Another option would be an I2C EEPROM. That would use your two pins, no worries about voltage drop to the ground, and could be easily reprogrammed if you need to re-jig a board to be another variant. A 24C01 1K bit EEPROM costs about 20 cents in small quantity, so it's not extravagant, and you could accommodate serial numbers and batch numbers, production date as well as board identification, if so desired.