Can an ESC-rectifier combination be used as a replacement for a buck converter?
The peak voltage of an ESC is not going to change. The effective output voltage is changed by changing the duty cycle. If you rectify the output you will have a pulsed output with a constant peak voltage. If you put a capacitor on the output, you will have a constant voltage that is comparable to the input voltage. The bottom line is that an ESC is not going to be very good at powering much of anything except a motor. It might also work as a light dimmer for incandescent lights.