So I was thinking about the transom gate valve and ways to make it more "inherent" in case I forget about it, and it occurs to me that a standard rubber flapper valve mounted inside tank opposite the gate valve would do the job. Essentially, you'd have the gate valve outside the tank on the transom, and the flapper valve mounted on the inside of the tank opening.
Here's a link to the type of valve I'm talking about:http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?productId=94242
A flapper valve will allow ballast water to flow in at full speed, but only dribble out if flow at all unless you push the flap valve in manually with your finger or a stick to drain the tank. The weight of the ballast water pushing back against it keeps it closed. It's essentially a backflow preventer.
In combination with porting the air-vent to the chain locker, this would make a Mac in the water inherently ballasted if the gate valve is open. Of course the exterior gate valve should also still be used.
You would have to manually decide to empty the ballast, which for many occurs only when trailering.
Boat is inherently ballasted and inherently safe whenever its in the water.
Difficult to dump ballast at sea
Must remember to dump ballast while trailering
Likely to get wet dumping ballast