Assuming you’re using the MOSFET as a switching device, it is possible to control the LED brightness by providing a PWM signal that you change the duty-cycle of (i.e. rapidly switch the signal voltage between its high and low levels over time, where the proportion of high time determines how bright the perceived light is).
ArduSub’s PWM outputs can be used for that, but you’ll need to adjust the minimum and maximum pulse durations of your lights output pin (using the SERVOn_MIN/MAX parameters) to provide reasonable duty-cycles for the PWM frequency (which I believe is set using RC_SPEED, but that applies to all the outputs, including for the inputs to the thruster ESCs).