When i completely stripped the old anti-foul off a Vega keel, i found a plug like that, it drained anything from inside the encapsulated keel, presumably the Rassy does the same.
I know little about Rassy's but I think post 2 sums it up.
The ballast is often encapsulated forward and that area aft will usually be the bottom of the bilge. In areas where the bilge water could freeze it is an excellent idea to drain all of it away when on the hard, It has the added attraction of making it easy to flush and clean the bilge easily.
This is very common in Scandinavian boats which are hauled out for the long winters and left out in freezing conditions. Everything is drained including the bilges to prevent damage.
My Twister has an encapsulated keel but does not have plugs like that.
I assumed (hoped!) that the keel is entirely filled with lead scrap and all voids filled with cement or resin so there is nowhere for water to collect (apart from a well under the engine, which can be pumped out using the bilge pump.
Interesting.
In the Twister the ballast is forward. Aft of it is a moulded-in water tank but I'm not sure if that takes up all the remaining space.
I'll have a look at the designer's drawings.
My LM (Danish built) has a screw in drain plug in a similar position which drains from the bottom of the bilge sump. (Or at least would if I could turn the screw-in plug!)
It has nothing directly to do with encapsulated ballast: you could have a similar arrangement with a bolt on keel.