As a further off-the-wall thought. Create a PCB with a matrix of X-Y connections, say about 8 * 8 wires. Provide the AVR with a 16 way connector, 8 pins to each of two ports. Use a different connection matrix on each PCB issued - just solder bridges across some of the junctions. That gives you 256 possible different patterns. Then scan the matrix like a keyboard to recover the connection pattern. Issue one to each person.
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