So invasive genetic engineering used to express the magnetically sensitive (instead of light or ultrasound or whatever) ion channels in the neuronal populations of choice.

Cool stuff, another tool, but there is a way to directly control brains using only magnetic fields and no invasive genetic engineering: transcranial magnetic stimulation. It's where a very, very, very fast changing magnetic field induces a voltage in a tissue. The rate of change of field being proportional to the induced voltage. Generally this means ramping up from 0 to ~5000 amps in a small magnetic coil in less than 10 ms. Unfortunately TMS is not very focal and engineering limits mean the effected tissue regions are about ~5-10 mm spheroids in general (with a slow fall off over large scales); too big for mouse models but fine for humans.