Hi Brian. Although I will be going with the interior valve/piping design, I like the idea of an exterior valve because of the ease with which you can put a BIG valve on there, making the flooding stage quick.
Not having really investigated the design much, a couple of thoughts come to mind.
First off, you gotta have through-hulls to operate the ballast system. One to put the air into the tank, and another to control the valve. It could be possible to have just one through-hull ( say a 1 inch ) and have two lines ( say 1/4 inch ) running through the nipple/through-hull with epoxy sealing it all up.
The two lines could be: one for air into the tank, and the other a hydraulic or pneumatic line to operate the exterior valve. The valve could be pretty much any size. 3 or 4 inch would be plenty.
Now for the valve itself. A butterfly valve seems to me could possibly leak as it gets older. I haven't much experience with them, but generally don't they have a central shaft where the disk pivots and when closed one side swings out while the other side swings in ? I think a mushroom or "poppet" valve would be less inclined to leak, where there's a disk that pops up into the seat. This type of valve could be fabricated easily and save a bit of money over buying an expensive butterfly valve. The main difference is, a butterfly valve needs power to hold it closed, where a mushroom valve is held closed by the pressure behind it.
I have seen a butterfly valve where there's two halves hinged together and both sides swing out against stops that seal, but the mechanism that opens the two halves is like a shifter fork and is bound to give you trouble is there's any pressure involved. I think Vance was going to use a mushroom valve type system, and he may be able to provide a link or better information than I can. Let me know what you find out, and let's get together for a beer again soon. Frank D.