Everything else is at Sha Tin though so it's only the x country they have to travel for. I think the horses will cope better than we think, as long as they are acclimatised well first and all precautions are taken to avoid heat stress and dehydration.
I so wish I was going! I cycled round the outside of Sha Tin 3 years ago and it's a pretty spectacular racecourse.
I'm so old I can remember the concerns about the LA Olympics (1984), smog, heat, distances... the 3DE was held over 5 days, 1: Dressage, 2: travel to XC location, 3: XC, 4:travel to SJ location, 5:SJ. The heat/smog didn't seem to do anyone any major harm, Toddy and Charisma won the individual gold...
I think the distances in question at LA were quite long. The XC venue at Beas River is only 30min from Sha Tin, and the transport is in air-conditioned lorries provided by the Hong Kong Jockey Club. Less stressful than the travelling many horses regularly experience for a one-day event in a British summer, I'd say.
Green with envy.... I love HK - lived there for two years and revisited 3 years ago. And to be able to combine HK with horses.... heaven! Hope you have a fab time