It took a lot of tinkering, but I've finally performed the free will experiment using multiple electrodes on the scalp.
The hardware setup was fairly simple. Take two more Heart/Brain shields, switch C7 for a 47µF cap on both, and stack them on top of the existing setup using headers. Make sure each shield has a different channel selected by shorting a different pair of pins on the analog out. These BYB shields are like legos for neurophysiology!
Modifying my MATLAB code to accommodate for three channels wasn't terribly hard. I've updated github.