First, let me say that I really like the idea behind your product - many good design decisions. It seems almost ideal for my project, so I plan to buy one soon.
As far as I can tell in looking at the schematic and library info the only ADC reference possible is the internal band-gap. Am I correct?
I need to take a measurement that is ratiometric, so it would be better to have an external ref derived from the sensor supply. I suppose I could use the internal reference to measure both supply and sensor, but would prefer otherwise since that would introduce another source of drift in the measurement.