First, congratulations on your new-to-you Mango! And double kudos for seeing this as an issue. I am by NO means an expert on Mangos. I have never been on one, so feel free to dismiss everything I say, I will not be at all offended, but I can discuss this part of Amel's philosophy of boat building and design.
With a boat this old, you can never assume that everything you find is "as-built."
As far as I know Amel is unique among yacht builders because when they decided that a compartment of the boat SHOULD never have water in it, they treated it as if water could never get in. In "traditional" yacht design it was considered essential to always supply a path of water to travel to the bilge sump. This makes having watertight compartments extremely difficult, if not impossible. Amel looked at this and said, "There is no way for other than trivial amounts of water (from condensation, for example) to get into the aft cabin, the saloon, or the forward cabin." Based on this, they did not set up the boats to drain to the bilge from these areas. While there are are certainly downsides to this approach, there are a lot of things to recommend it too. First and foremost is if you find water in any of these normally dry areas, you know there is a leak that needs immediate attention! If it just drained to the bilge and was pumped overboard, you might never notice until it got REALLY bad.
I see two possibilities with the situation you describe. Amel MIGHT not yet fully committed to this philosophy, and "cheated" on the Mango, OR a previous owner made a modification to the boat. If I had to bet, I'd go with option 2. If someone had a significant amount of water leak in past the rudder packing (for example), they might have thought it best to just cut a hole and have it drain, instead of fixing the problem. In the Amels I have inspected, cutting holes in the watertight bulkheads by some one of the previous owners is among the most common things I see that show a lack of understanding of the "Amel Way."
I believe that Amel assumed that an inspection of all the bilges in the boat was a routine, at least daily if not every watch-change. They assumed if water was found where it should not be, the problem was fixed at the source, not just the symptom. This strikes me as just good seamanship, and common sense.
As always, my opinion is worth what you paid for it...
Annapolis, MD, USA