Thank you @teaderechoweb and @wizard – The finished project is written up here – Wearable radioactive labcoat finished project + tutorial (I didn’t actually wire it quite like the diagram above, but I did heavily use the pixel map tool!).
It was a fair pain in the butt to get a single image with each LED visible (I had to hand-mark them all, and then photoshop two images together), but the map tool worked a treat once I did so).
Note: I first tried to just get a single image with all the LEDs “on” but because of the inconsistencies in the wiring and the quality of the diffusion, that wasn’t precise enough, so we went with the approach in the other post.
Thank you!