Having created a pcb from our earlier design, we've finally had time to sit and work out the pinout. And this time we're delighted to report that it actually works. Each pin on the 24-pin connector performs just one task and the correct combination of power and ground onto the pins causes just one LED to light up at a time. Result!
Here's the pin-out from the earlier pcb. The pins along the bottom need to be pulled to ground, while the pins along the edge should have power applied to them:
So if, for example, we put power onto pin2 and pull pin 23 low (to ground) the single LED in the top left corner of the matrix should light up. Repeat as necessary for each individual pin and we've finally got a fully working, individually addressable LED matrix. Yay!