Some of the sports drinks manufacturers (e.g. Pocari) also make a dried packet version of the drink, which can be made up using water. I wonder whether it may be worth the tour leader or participants on the trip taking one or two of these packets on hot days, so that the drink can be made in case of emergency (of course it would still be necessary to carry water). The drinks machines at the Mai Po car park also sell bottles of Sports drinks, as do many of the vendors selling drinks at popular walking locations.
It is important to remember that in hot weather, you will be losing salts through sweat. It is important to drink enough liquid but it is also important to replenish the salts. One tip is to check which type of bottled water you buy - mineral water contains some mineral salts, and is probably better than distilled water on a hot day. It's also a good idea to avoid caffeine if possible (e.g. cola).
