Thanks to my multimeter and some advice from bbraun, I can confirm the IIci brings out pin 2 of VIA1 to pin 7 of the diagnostic edge connector with a 100k pullup to +5V. For people really interested, the 100k pull-up resistor is R110 on the bottom of the board near the edge connector. The edge connector pins are labeled 1 through 15 on the top side of the motherboard, so you'll be looking for pin 7.
I just tested his control panel and it does in fact control whether that edge connector pin is outputting 5V or 0V. So with that you could definitely control something. As you point out, an RGB LED is going to need more control pins than 1, but it's a start!
You're right that you need 3 bits, but even then, as a digital I/O you're only going to get 8 possible colors. Each of the R, G, and B can be either on or off. To do a ton of different colors you have to control the intensity of each of the 3 colors which would require something like PWM, which would be a bad idea to implement in software in this case IMO. I wonder if there's a 1-wire PWM controller IC that could do it for you.
I just tested his control panel and it does in fact control whether that edge connector pin is outputting 5V or 0V. So with that you could definitely control something. As you point out, an RGB LED is going to need more control pins than 1, but it's a start!
You're right that you need 3 bits, but even then, as a digital I/O you're only going to get 8 possible colors. Each of the R, G, and B can be either on or off. To do a ton of different colors you have to control the intensity of each of the 3 colors which would require something like PWM, which would be a bad idea to implement in software in this case IMO. I wonder if there's a 1-wire PWM controller IC that could do it for you.