So, I got tired of waiting for someone to take some photos of their AV kit and just probed the board myself.

The AV kit is a SCAM. You're paying $250 for a breakout board.

The pinout of the JP1 header is:

Top row (Pins 1-5)- Ground
Pin 6 - Composite Video/ Svideo Luminance (Y)
Pin 7 - Svideo Chrominance (UV)
Pin 8 - Microphone In
Pin 9 - Audio Left In
Pin 10 - Audio Right In

I didn't probe the JP4 header for "balanced" very carefully, but:
Pin 2, 5, 6, and 9 are ground, and it just uses 2 pins for each audio channel.
Left is likely pins 1&7, right is likely pins 4&8

For $20, you can get a DB9 to Motherboard 2x5 pin header bracket, female Svideo/Composite/L/R cables, and a DB9 connector and hood and make a custom breakout cable.