OLIVER Stone breezed into town last night to spend a couple of nights at the Regent. But the director of JFK and The Joy Luck Club was not giving too much away about his activities. ''This is just a personal trip,'' he said, letting slip the fact that he was also intending to do some low-key negotiations about a new series he was planning to film in the territory. ''I can't say much, but it'll be a TV thing about life in contemporary Hong Kong. It's a secret right now . . . but we might be able to announce something in February.'' He is here with his Iranian assistant, Azita Zendelhal, and said he would be going out with an old friend, Clive Ng, ''and maybe meeting a few ladies''. Actress Hiep Thi Le, star of his latest film, Heaven and Earth, is due to fly in to Hong Kong today - the first time she will have been here since she arrived on a boat as a Vietnamese refugee. Stone said the next movie project he was working on was called Natural Born Killers, which he explained rather mysteriously would be ''a dark comedy about mass murderers''. He said making that subject matter really funny would be quite a challenge.