Some good progress over the last few days. PCBs arrived, and I've assembled them - cut down a bit, I left out the bus switches for two reasons:
1. they aren't needed with only one input & one output.
2. I didn't have them.
So the first modules are essentially a weird looking Raspberry Pi add-on. One mistake I did make - no +5V. My backplane spec didn't include 5V because most parts I'm looking at using are quite happy at 3.3V. But maybe that was a mistake. Anyway, easily fixed for now with a piece of wire, and music now comes out of my DAC!
Next: I2C comms from the Pi to the IO Expander / EEPROM chip that will eventually serve as module ID & control.