Jack,
When you need to externally input SPI signals onto P14, you'll need to remove a component. It is true that +5V_USB will power one side of the isolator chips, but AVDD will power the other side at VCC2. THis will force the outputs of the isolator to the unknown value of the input. This is undesirable.
Ideally, we would put the En pins to GND to guarantee that the outputs are high impedance, but the EVM doesn' t allow for that easily. I suggest that we remove FB17, which will remove the power supply from the output side of the isolator, and the outputs should turn off. However, the output pins are not necessarily high impedance in this state. The AFE pins are 20kohms to GND. We will test this in our lab as well.
Thanks,
Chuck Smyth