https://cimg8.ibsrv.net/gimg/www.off...6260cb2c7f.jpg
Could you not also run hose from the block drains, manifold drains and the sea water pump drain down to a common manifold similar to the Mercruiser system.
https://www.ebay.com/i/123060649398?chn=ps

Yeah, it was a sweet set up my old mechanic buddy suggested... They are only ~1.5 to 2" long..

The thing I liked about the ones I installed is you were still able to access with an awl to poke away and metal/rust build up...

Mine cost 3 bucks and work real good.
The reason I first did it was because the threads in the block I had got screwed up over the years. After the first winterizing I did it to every motor I touched. Straight run so easy to poke through. Brass on brass and if something is missing its a quick trip to Depot to replace.

https://cimg7.ibsrv.net/gimg/www.off...232ac360bc.jpg

Its a great idea but how are you going to stab all engine side drain holes to clear them from rust - scale - sediment - sand and so on. Thats a crack block waiting to happen down the road. These basically are the same as the OEM Merc single drain point system - one valve set up to drain the water from all those sides. That does not work down the road because you have to unclip at the block and stab the crap out of those drain holes to clear from rust and scale and sand so the water can drain out of the block. Plus other areas get so plugged up as well.

Mercruiser has a single drain point system, it has been out for 15 years give or take on the years. Anyone can buy that. One knob drains all, until it gets plugged with the usual stuff. When that happens it does not work so good.

no matter what drains or extension you put in water drain holes you still have to stab it to make sure its clear so the water can drain all out. And on the ball valve you still have to reach with your hand to manually move the valve to open it up.. One still needs the access and reach for that install. With that said if you do not have the access to install it with the engines in the boat how are you going to have access to open up the valve to drain with the engines installed for winterization ? Just asking

Not picking on your post, just using your valid point here.

In the past when I did have to go thru the drain the block at seasons end what I always did several times during the season while sitting in the slip was to pull the drain plugs and run the engines, just to flush sediment, much as possible out of the fittings/block. Honestly in many years I don't recall ever having plugged up drain fittings at the end of the season.

Closed cooling is a godsend come fall. Couple minutes and four of those little blue wing plugs and that's all she wrote.