Closed loop is tempting but if you still have to winterize then its not tempting, definitely looks like a lot of work and parts to source if they are even still available. I do worry about corrosion, its not too bad on the lake I use. I know places with more reactive minerals in the water can have big problems over time with aluminum block and heads etc.
I have thought about in the past a petcock fitting on the block drains, some kind of petcocks on the exhaust manifolds and a dump valve in the oil cooler line (lowest point on engine) there might still be water under the thermostat and in the heater and shower, ballast (if installed) AND in the sea strainer and the V Drive cooler (lowest point in boat) cooling system besides inlet)
So not good for a hard freeze.
The wet block method is messy but could be streamlined with a fitting in the water supply line to add coolant at the inlet of the sea strainer, a Y valve. If you used the "safe" antifreeze, you could even do it in water, run heater and shower, it would go into the vdrive cooler etc. You could then pour some into your ballast tanks via the vent hole, run the drain pump and you should be set for whatever. Not sure what the rules are regarding discharging the "safe" antifreeze into freshwater lakes, but it would be nice to quickly winterize then take the boat out on a warm day, rewinterize again etc. Hmmm that could work. I'm getting ready to put my boat in the water (on a hoist) year round so this subject is very interesting to me.
In the past, I have always trailered and the "dry block" method seemed to be the easiest, not the quickest in a D drive. It's a lot harder on a V drive, with shower, heater Vdrive cooler etc so the "wet block" method is making more sense to me these days for my SX. Plus, you are only opening one connection, I have made mistakes in the past and forgotten to hook up a hose or tighten a clamp doing the Dry block method.
