I have never even heard of anyone's eggcrate getting blocked up. Gauze or similar materials like fibreglass screen would actually be more likely to get clogged, due to the smaller holes. And what's more permanent than plastic?

What I keep under my terrarium, in the stand, is the bucket full of RO water for the misting system (one way of making rain), extra lightbulbs, a towel or two for cleaning the glass, a small toolbox containing extra rocks, driftwood, aquarium silicone, airline tubing (for no apparent purpose), and extra tubing and other components of my misting system; and if my terrarium was more accessible (it's against the wall, behind the kitchen table, so it's actually quite a pain in the butt to try to get stuff out of the stand cabinets), I would also keep a misting bottle for manual misting. I also have all of the outlets that I need hidden down there, so that there are no cords showing out the back sides of the tank and stand. Oh yeah, and if my tank wasn't right next to the kitchen, where I keep scissors, I'd keep a pair in the toolbox as well, for trimming rotting plant material, etc. Chopsticks can also be handy, as can fishing line or wire for attaching new epiphytes to stuff.

As for rain, a misting system will not be able to run off of any water supply connected to your tank, because the misting heads would quickly get clogged with the miscellaneous debris that gets into the water. I've heard of something called a pneumatic rain system, but have also heard that these often don't work very well. A hydrofogger would mimic fog (duh), which isn't exactly rain, but has the same effect of 'watering' the plants and keeping the humidity high. 'Rain walls' are quite common, and can be run off of tank water. They are basically a tube with holes in it across the top edges (but not the front) of your tank. You could have it running constantly, or put the pump on a timer (probably smarter, because very few plants want to be that constantly wet). How exciting that you're nearing completion of the design process! It's really great that you're putting this much thought into it. I wish I had. . . live and learn! =)

Alex: I think you might be confusing egg cartons for egg crate lighting mesh. It's hard to imagine 1cm square holes clogging :)