ONE of the engagements just entered into the ever-bulging Keeping Posted social diary (a testament to consummate consumption, as it has been referred to at times!) is an invitation to a gala occasion at Macau's Hotel Bela Vista next month. The apres -dinner entertainment in the romantic setting of the verandah will be provided by the Hong Kong Ballet. The event, billed ''a unique evening under the stars'' will be set alight by, what promises to be, a spectacular fireworks display. And a few tickets priced at $750 are still available. But we do hope the choice of work by the Hong Kong Ballet for the evening's performance - Who Cares? - has no connection whatsoever with the reason for this special celebration, 1999 notwithstanding. It is being held to mark Portugal National Day.