Why not go to school on the experts? That is, the guys who ride in extreme heat all the time. In the US we have a lot of desert and in our southern region, summers are very HOT and HUMID. We ride year round.
If you've never experienced SERIOUS humidity, then you may be in for a shock. Think survival mode.
Parts of Africa, Asia, much of Latin America can have serious humidity in their summer/rainy season's. So it's not just high heat you must deal with....its
debilitating humidity.
Check out what racers are wearing and use them as a guide.
In heat I wear a mesh/convertible padded Joe Rocket Alter Ego jacket with only a Moto X jersey under. This jacket has elbow, back and shoulder padding which is placed close to the rider (this is important!).
In super heat I simply wear a Long sleeve moto X jersey with only elbow pads.
Sometimes you have to ride at risk. TIP: Ride carefully! RTW riding is not for sissies!

Soaking the clothing down is good but uses a lot of water. Cool vests are OK but mostly a cotton shirt works OK too.
Pants are a much easier solution. Moto X pants....high quality ones used with good knee pads, inner leg leather patches, and venting. These pants are quite good. I also wear a kind of bicycle short (EVS) with high quality hip and tail bone pads (foam) under my MotoX pants.
Moto X pants are vented, quite versatile and you only need a pair of light rain pants to stretch there window of operation much further if it cools down.
No expensive riding pants will match Moto pants in the heat. In normal temps I ride in leather pants.
I'm not talking about kit to ride round London or Scotland in. This is strickly
RTW type gear for hot weather...really hot weather...yet still some protection.
Also, be sure to plan your trip as best you can to avoid the hottest weather, worst rain, and coldest areas. Knowing world wide seasons and weather patterns will save your butt. I see so many Brits who come to visit the US at the WRONG time of year in the wrong place!
Good luck, this is not the hard part! It's the fun part!
Patrick