CONSIDERING she lives on a pig farm in Tai Po - as she never tires of telling me every time our paths cross - Aileen Bridgewater is better qualified than most to run a ''Charm School'' for engineers. The diminutive but high voltage image consultant set up shop in an elegant villa just outside Shanghai where engineers from China, Hongkong and the Philippines needing to brush up on their charm assembled for the three-day seminar. But engineers aside, it didn't take long for Aileen to sniff out juicy local gossip. ''The villa where we were staying was set in an idyllic estate where Chairman Mao, so I was told, kept a concubine,'' she confided. Aileen, usually the soul of discretion, added: ''In fact, the bed that I used was believed to be the one the Great Helmsman himself had slept in.'' There was one minor hitch when Aileen - a martyr to insomnia - got to the villa. She had forgotten her sleeping pills. Fortunately, a copy of Chairman Mao's Red Book expounding his weighty thoughts found by the bedside table took care of that problem. ''Why, you started reading it and fell asleep?'' I ventured. ''No, no, my husband hit me over the head with it and knocked me out,'' roared Aileen. The seminar, at which the topics discussed included one mysteriously entitled ''The Telephone is Your Front Door'', did pose some communication problems for Aileen. Her advice to her avid listeners at one lecture that they should ''always remember to wear a smile'' was met with blank and puzzled looks by certain delegates from the mainland, one of whom earnestly inquired: ''Wear a what?'' Poor Aileen then got involved in such a long and unwieldy discussion on what exactly she meant, that she found herself regretting having used the phrase in the first place. Charming, eh?