Paulware:
Well done I have a friendly robotics lawn mower that I would like to hack with your approach.My lawn mower starts to spin the wheels when it tries to go up a hill steeper than 15 degrees...It would be nice to detect slippage of wheels and change direction when this is detected.
Does your yard have many hills? Maybe the weight/wheel chair wheels don't have this issue
I've got one of these, but I haven't taken the time yet to hack it (purchased it used off of Craigslist for $50.00!); surely there is some place on the motor driver board that is monitoring (or can be made to do so) the current to the motors; look between where the motor wires connect and the output h-bridge drivers - you might have to do some probing. Basically, you are looking for a point on the board where you can monitor a voltage representing the current consumption of the motors; it will be lower when the wheels slip. Good spots to probe for are any place marked "TPn" - where "n" is a number (stands for "test point" and is used for troubleshooting the boards, usually). Testing will be another thing (you'll want to build a pony brake with adjustable slip, and test the motor without the gearbox).