If it is true subfloor ballast, I believe they would have to split the boat at the rubrail and put it in under the top cap. Just a guess, I havent been to the factory to see how they make the boats.
On a side note, does Moomba have any boats with true subfloor ballast? The Max and Kaiyen that I walked through at the dealer I could lift seats or bins and see the ballast bags. I havent had the pleasure of seeing a Supra in person, but my understanding is they have true subfloor ballast. I have been on G and GS nautique and that is what I think of with subfloor ballast. When you open any compartments or seats the there is no visible ballast bags.