For those who are adventurous, Surplus Center currently has some hall effect foot pedals listed. They are take offs from Raven electric mowers: http://www.surpluscenter.com/New-Arrivals/FOOT-PEDAL-DRIVE-BY-WIRE-WITH-5-VDC-HALL-EFFECT-POSITION-SENSOR-11-3493.axd.
I don't think the Alltrax DCX controller can use hall-effect sensor input. But, I know there are others that can.
Some day I will get around to my next tractor build. Plan to try a B&S E-tech motor with the British controller (Quad four, I think). I believe it will accept the hall-effect sensor input.
At $30, it might be good to have on hand to experiment with....