I'll try answer some.. I don't have one so I could be totally wrong!
All the pins are exactly the same besides the last 3, they're going to be the ground for each color LED used.
The problem with the sparkfun backpack is that it uses a normal LED, the LCD has 2 extra pins, it uses a common anode LED for the back light. What that means, is the GND pin of the new LED's, will be where the PWR of the old LED was. And to control all the pins, you're going to need to sink the current, I believe the best way is using some transistors.
But yeah, it's very cool! I just wish they used common cathodes, seems like it would be easier to control with an MCU :
If I see one with a common cathode, oh I'll be snatchin it right up:)