Belly heat, RHP's, or just keeping your snake room in the 80's it's all the same in the end. As for your humidity issues take a tub and use a soldering pen to melt some holes in the tub to get some ventilation going. I would do just a few to start and see if that does the trick, if you still have visible condensation on the inside of the tub try adding a few more holes till you get it where you want it. Can't say I've ever seen anyone use rocks in a tub set up before, I would think it will make spot cleaning a pain and I wouldn't be surprised if the snake just pushed them all aside anyway to lay on the floor of the tub. I have two racks that I run and heat tape is so easy and virtually worry free, just don't skimp on a thermostat to regulate it and check your temps occasionally with an IR temp gun.
Just my $.02 (or 2 shillings if you prefer)