The brake assembly will rust if it remains wet. If you flush it, then drive a decent distance, the heat from the brakes will dry it out. That minimises the problem.
We trailer sail, so we travel at least 300 yards once the trailer has been flushed. We have absolutely smothered the bearligs in grease and I regularly spray WD40 into the hub. I know, oil and brakes don't mix.
The hubs are rusty but the brakes still work.
Another tip, never leave the trailer brake on!
The answer, I firmly believe is that there is no answer!
I did hear of someone who merely removed the entire brake mechanism, but left the cables in place so that all looked well! I can imagine this is a good solution until the DOT investigates a multiple pile up caused by a boat trailer overtaking the tow car!