Let's start with some sort of rainjacket. I'm not sure I'd fool with a ponchosome people swear by them, but I think they're heavy, clumsy, and uncomfortable. I don't know where you've been looking, but the vast majority of rain-specific jackets I'm familiar with don't have "liners," at least not in terms of insulated liners. What you want is a decent waterproof-breathable jacket with lots of ventilating zippers. Marmot's Liquid Steel ($375; www.marmot.com) is a light, well-ventilated jacket that uses the newer, very breathable Gore-Tex XCR formulation. Another choice would be something like L.L. Bean's Stowaway Parka, just $175 (www.llbean.com).
But are you going to feel hot wearing these in a rainforest? Of course. They're not water-cooled, after all. If it's 90 degrees and 90 percent humidity, no man-made garment on earth is going to feel particularly comfortable. A rainjacket's fine when you're sitting around, but you'll be miserable if hiking or otherwise on the go. I'd recommend you pack lots of light, synthetic, fast-drying clothing, stuff like Patagonia's Capilene Silkweight T-shirt ($32; www.patagonia.com). That, and an umbrella. Even a few umbrellas. Under the forest canopy, not that much water should be hitting the ground, anyway. And in open terrain, where an umbrella will be more maneuverable, you'll get good protection from the sun OR rain.