If this helps at all, I have carried a Ute, Umlindi, and a modified hybrid frame C20M (old, discountinued HPG packbag) in hot / humid and hot / dry environments in Peru, Colombia, Paraguay, and South Africa. Evan hits the nail on the head....hot & humid sucks....no getting around that. When I've carried my Ute, I simply adjust out the load lifters on most terrain so that the pack has some space for my back to breathe. When faced with steep terrain, I would pull the load lifters in taut, to have the load close to my back for the added agility. While using my Umlindi or the C20M (which I have set up as a sort-of extra big Umlindi), I always use these packs with a belt (Recon or Prairie). The load I carry in my Umlindi out in jungles isn't super heavy (under 50 lbs). When faced with a long slog in hot / humid jungles carrying this pack (and the C20M, as well) I adjust the shoulder straps so that they are a bit on the loose side, but I keep the belt cinched taut, and I also make maximum use of the delta straps coming off the sides of the packbag down to the belt, in order to suck the load into my lumbar and hips. Because HPG packs have that wonderful yoke-style shoulder harness, I still have decent stability while keeping most all of the weight on my pelvis. With the Umlindi especially, I can often carry it like a "tall lumbar pack", in that once I have it dialed in to my body just right with my Recon Belt docked to it and the delta straps engaged, it will carry like I barely even need the shoulder harness. This lets me have some air flow to my back. As with my Ute, when I get into technical or steep terrain I tighten up the shoulder straps to bring the pack closer in.
Hope that helps.