HONGKONG might be losing its position as a world producer of fake designer goods, but Keeping Posted can announce that at least one of the territory's less successful forgers has moved into a very specialist area - chamber pots. When The Ritz in London recently sent out a worldwide appeal for memorabilia, its post box was crammed with ancient dressing gowns, bath towels and soap holders that had been smuggled out in suitcases throughout the century by ''forgetful'' guests. Then, by express mail from Hongkong, came a rare find - a porcelain potty in excellent condition, with the hotel's crest emblazoned on the side together with the date, 1904. It did not take the Ritz antique experts long to smell a rat. The hotel did not open its doors until 1906.