Re: Charlie's land based flush system: I realize this won't help anyone at the dock, but I use something similar. It's an old poly barrel, cut to height to allow water (when filled ) to cover the cavitation plate (ie all intake ports). I added 4" casters to the bottom and sealed the bolts/holes with 3M 5200. I add Salt away to the water and store the barrel full until next needed, replenishing any amount that's evaporated with new salt away. I change out the water every 3-5 flushings. This system is worth every bit of cost and effort to make. Flush muffs are a pain spew water all over the place, don't allow easy injection of Salt Away (the adapter is junk IMHO) and I don't think provide enough water to the system.
Another idea: I routinely stop off on the way home from a salt water trip at a fresh water launch ramp. I drop the rear tie downs, loosen the winch strap and back the trailer and boat in. I start up both engines for 5 min. This also washes off the back of the trailer and wheel and brakes...
As to dock flushing: here's what I would try. Use a hose fitting which screws directly into the engine. (This can be done with engines trimmed up. The port is usually just below the cavitation plate, next to the top gear case vent screw. Lower the engine (agreed w/your mechanioc running the engine in the tilt position is not a good idea. Granted the engine will still suck up some salt water, but I believe this will still help. Trim up the engine at the end, with water still going, and kill it when it's up.