How about splitting the lights and number plate.
On my (and most, I am guessing) trailers it should be possible to mount a small board either side on the extendable trailer board arms.
On one side have the number plate(square type) and the side/brake and indicator, reversing and number plate light, and on the other side the same but without the number plate light, and both sides have the reflectors.
That way, they can (probably) be retracted to be out of the way of engine etc for launching, and can be left if they are submersable.
Just a thought, and there is probably something illegal about it, but worth a try if it is allowed?