The only time I have used the RDSon resistance of a MOSFET was to roughly measure the inductor current in a boost converter, several 10s of amps.
I have also seen it in switchmode converters as a means to detect a fault whithout introducing an extra resistor hence achieving high efficiency.
For any reasonable precision I would advise against using RDSon as it varies greatly. You can possibly use a low shunt resistor and an instrumentation amplifier, which you can get in a black box from the usual manufacturers.
Hall effect current transducers could be suitable, Allegro comes to mind. Careful with stray magnetic fields.