Is Arduino Mega Hackable Over The Air
the bootloader doens't listen on RX/TX3.
to start the bootloader reset is required
to communicate with the bootloader a serial programmer must have STK500 protocol or a telnet bridge for avrdude, which your NodeMCU sketch doesn't have

If your NodeMCU sketch has OTA, then the sketch could be updated to an avrdude telnet bridge. then reset would be needed to activate the bootloader. If your NodeMCU doesn't have a wire to Mega reset pin or you don't have a command in your Mega sketch to activate the reset, some serial overload of your Mega sketch could be a way.
But all this doesn't have an effect because the bootloader only listens at RX0