My bilge drain plug is epoxied over, so at the end of the season, I used a small hose to reach past the fuel tank and sucked the last bits of water out. I then poured in some RV antifreeze and pumped it out with the electric bilge pump that lives under the fuel tank. While checking on the boat from time to time over the winter, I would turn on the bilge pump, it was dry in the bilge.
My boat is a 1973, probably a series II, the keel is solid forward of the large sole cover. A diesel tank fill up most of the hollow aft section of the keel. The tanks sits on a inverted ushaped piece of fiberglass mat/cloth. I glassed across the bottom of the u to form a box and installed a narrow whale bilge pump inside on the floor of the box. I would guess the area under the fuel tank would only hold about 3-5 gallons which is only 40 pounds of water max. Not much good for a water ballast.
I am guessing that Per Brohall didn't fill this area of the keel for several reasons:
1) the boat probably doesn't need it (see the discussion in the manual about using iron vs lead ballast
2) there is probably something to do with balance of the boat, the heaviest part of the keel is under the mast
3) Brohall probably wanted to keep the boat as light, but still wanted a longer keel and keel hung rudder, so the void is there to provide hydrodynamic shaping
4) gives a good low spot to install the diesel tank and forms a low sump to collect any trespassing water.
5) some other reason I don't know about (Brohall's girlfriend wanted a cool spot to store many bottles of wine?)
-Tim