Updated: figured it out
I can't make sense of the pin mapping on this Xadow board to read analog values. Note that analogRead() has funny behavior to start with. But I looped through pins 18-29 (A0..A10 in pins_arduino.h), and the only one that came up with anything on analogRead() was 23, which seems to correspond with the pin labeled SCL on the breakout board.
I can't find anything that would explain how SCL/23/A5 relate to each other, or what other values would even be sensible to try. There's a pin labeled A5 on the board, but pins_arduino.h #defines it as 23, and somehow that ends up on the pin labeled SCL.
At least I found one pin on which I can read the ADC.