Yes it would be possible to calibrate nonlinear extrusion using the filament monitor. This is unfortunately one of many things that I don't have time to implement right now. You could do it semi-automatically like this:
Create GCode files to print, that extrude into air at various speeds while also moving the print head in the XY plane. At the start of each file, put the filament monitor in calibration mode (M591 Pn S0). At the end of each file, use M591 Pn to report the calibration parameters, which includes the measured/commanded extrusion percentage.You could combine all these files into a single file, using M226 to pause between the sections (which restarts the calibration).