Possible to measure EEG signals with an AM receiver antenna/circuit?
The wavelength for EEG is 5 orders of magnitude greater than AM radio signals so the practical signal capture methods of a fixed capacitance being dependent on the gap to the live subcutaneous skin over the skull  might be less sensitive to motion variation but even less sensitive to signal pickup.

Considering ambient noise of power lines is also 5 orders of magnitude greater than EEG signals, that would make the idea a show stopper.

Although I have done very limited testing, my understanding indicates , the best signals will be captured by ;


maintaining a firm cap pressure over the electrodes to minimize modulation of scalp pressure from motion artifact which creates a large modulated voltage from capacitance variation of a fixed charge ΔV=Q/ΔC
maintain a high input impedance and high CMRR in the cables as well as INA
maintain high quality shielded balanced twisted pairs using the Vcm as an active guard shield signal or use RLD signal as your Common mode active guard.