After much research and fiddling, I finally figured out how get the Twisted Pear Audio WM8804 board to do my bidding. The only thing I want it to do is convert I2S data to S/PDIF data. I had a hard time figuring out how to make this work because the user manual from Twisted Pear Audio only gives configuration information for converting S/PDIF to I2S. I initially thought I would need a microcontroller to configure the registers in the chip, but after digging through the datasheets for WM8804, I finally found a small section describing how to run the chip in hardware mode (no µController needed) and get the results I wanted. Below is a picture of my test setup using the RN52 bluetooth module as a I2S source, and a picture of the WM8804 board showing the switch positions for configuration.

The picture (left) doesn't do a very a good job of showing it, but in order to make this work, I connected the bit clock output of the RN52 to both the master clock and bit clock inputs of the WM8804 board. The S/PDIF output from the WM8804 board is then connected to a TOSLINK optical transmitter which is connected to a cheap TOSLINK to analog converter. This configuration works flawlessly.

In the picture to the right, you can see the dip switch positions. The dip switches, from left to right are up, down, up, down. This translates to low, high, low, high levels. 

I haven't decided if I'm going to use this board or not. Although I still really want to use an optical medium to move audio data across the vehicle, I'm not sure I want to go through the trouble of figuring out volume control.