A couple other people have suggested the idea of using a tent without the rainfly and pitching a large tarp over it to direct rain. I think that may be the best option, even the most well-ventilated rainflies won't be able to cope with high temperatures and humidity.

That makes my choice much easier, since there are several sub-5lb two man tents out there that look good. The tarp will probably be cumbersome, but should be worth the trouble if it works like I think it will.

Thanks everyone for your input!