Thanks for everyone's replies.
Responses in Order....
Ex-DC-Driver - Regarding why. Simple put, because it is safer. Some aircraft which were built in the 50's, had numbers that were very optimistic. It was very popular in that era to juice the numbers to sell aircraft. The Ac500 is a fine example. I can't remember the exact numbers but it says you can land at gross, in something like 1100ft. It also suggests that you approach at around 65 kts. Both of these are near impossible. At gross and 65kts, the aircraft is in a full stall, not just horn, full wing drop chaos. Maybe if you hit the runway at 65 you could stop in 1100, but you might not be able to use the aircraft again. But, according to the numbers, and the 'buffer' it is perfectly legal. But, I assure you those numbers are truely outrageous. The t/o numbers are even worse. Something like 1400 at gross. Absolutely absurd. Anyone who's flown one can verify. So, in my mind, not why, but WHY NOT. The obstacle is cleared, the runway is solid, and the is 50 feet of water 4ft of the end of the runway. Which do you really think is safer?
Spitoon - Regarding changes to the displaced portion. As you described yourself, this portion is clearly allowed to be used for backtrack, takeoff, and landing roll from other direction, it's just landing that's in question. If someone was digging a hole ON ANY PART OF THE MANUEVERING AREA, there would have to be a notam, and resultant markings. It is still part of the manuevering area, irregardless of it's availability for landing.
Still very interested in any quotes from law, or even guidance books such as AIP's. I've read the ICAO standards several times, but it doesn't clearly stipulate yes or no.
Thanks again for everyone's input.