Small update (still working on this!!):
Since the last update a few things have happened. I ordered the sainsmart dac based on the pcm5122 as mentioned previously, and (I think) proceeded to kill it. I have a new 5122 chip on the way. In the meantime, I have redone the i2s lines to the pcm5122, beefed up the power supply a bit with a little LC circuit made from parts laying around, and put it all inside the case (sans internal speaker). Even this little 5102 dac blows the stock dac out of the water... For less than five bucks! Will be very interested to see how the 5122 with vastly better power supply and analog components will do. The change will be small for sure, but maybe noticeable. We'll see if it's worth 5x the price of the little 5102 board.
Also ordered a WM8804 transceiver board, as the eventual goal is to get a digital signal out to a receiver or dedicated dac. It will be a little trickier as it requires a mclk signal, and I haven't yet figured out exactly what frequencies the Echo's various clocks are at. (If anyone has a decent scope and wants to poke around in their echo, let me know!)
A few photos: