jtw11:
The monitoring cell in a wideband sensor is maintained at 450mV by the pump cell, so adding an ohm or so there would offset the result into the controller - this however would be a known, and I could account for this in software. Unfortunately that's not the case with the calibration resistor.
Rethink this. Adding an ohm or so in series with the analog input, impedance of around 100 megaohms, won't make much difference.
If you expect to connect your analog mux some way other than in series with an analog input, I'd like to know what you're planning.