That baffled me for a few seconds, but I get it now: Where were you when I was originally putting this together?
At the time, I just visualised substituting electronically controlled switches for the twenty physical switches of an external keypad. The input LEDS of the optocouplers are indeed connected in a row / column matrix, driven by tables of values chosen so that only one optocoupler LED is ever turned on.
The optocouplers came from the obsolete part bins at work so they didn't cost me anything, but I do admire the elegance of your pared - down solution.