‘Unlimited’, but actually harvesting the rotating iron core’s angular momentum until it stopped and the earth was destroyed :D

Yes!

Question, 'tho — to what extent is the geomagnetic field necessary, adding, or diminishing the effect?

For sure, as the charged particles arrive, their paths are being bent by the magnetic field, so stealing energy from the core.

But to what extent are we 'harvesting' that energy?

Seems the flares would still create a gradient across dozens or hundreds of miles of wire even without the core and our geomagnetic field.

If the field is enhancing or multiplying that effect, then we can certainly be said to be harvesting that energy. But, if it is neutral or diminishing the induced currents, it seems just like a side effect.

Also makes me wonder how much energy is there, and how long solar flares would take to deplete it...