I switched to NiMH a few years ago and am so happy about the reduced waste.
But my charger has no discharge function, and if charging takes over 90m I must write down the MAH and restart the charger to start from 0. It can be frustrating. Your device would be very useful for me.
Resources left on the board are: One Arduino I/O pin (the TX one), one unused relay. That could be used for one of the six channels to decrease the load resistance slightly by splicing in a parallel load resistor. By measuring the voltage drop from momentarily increasing the load, the ESR could be computed, at least roughly, for that one battery position.