Hehe, always many ways to solve a problem
Why not design using an edge connector, with a built in key ?
Edge connectors these days are not cheap, and are trailing edge/low volume products.
- with a bonus irony, that more pins cost less than fewer pins.
eg PCI 36 way edge connectors do have a key, and show ~20c/500|+
OR Can you not design in a proper connector, cheaper than trying to key something manually ?