Someone noted that you can measure the voltage drop across a measure of wire. Yes, you would have to probably make that wire long to get a high enough resistance to measure.....but it is possible.
Now, why not just add a very small resistor in series....because it does not seem to be allowed. The rules say that the high impendence inputs of a circut may be attached to some number of points described, but it clearly indictates that this is OK because it does onto affect the circuit appreciatbly.
Placing a series resistor violates all the rules regarding how one MUST wire all the motors/spikes and other controllers......so it would be against the rules.
Still, the job is not as hard as it seems.....and with an op-amp or two (depeending on whether you need to build a true DIFFERENTIAL AMPLIFIER or if you can get a good common ground reference point).
There are of course other ways to do such a measurement using the magnetic radiation off the wire as current flows. I think this might be the better, more noise-free way to skin this cat.
-Quentin