yea I think it will work for awhile anyway. eventually it will break down but in the meantime it will give really good drainage. I would just skip the screen or instead use something organic like a layer of sphag. moss. My tank that has been doing well for around ten years is layered thusly: gravel/moss/potting soil/charcoal/peat & sand mix
so in my case the gravel won't break down but serves the same purpose that your bark would. another consideration is the types of plants you want to use. I say go for it.

It was going to be a fairly simple terrarium, mainly revolving around plants that are available -- ferns and moss, a kind of woodland look. Perhaps a miniature sinningia if I can find one. I could get some egg-crate and put it in the very bottom, before adding ReptiBark?

Brenda

I don't think the eggcrate would benefit anything. Seems that people use it for making a 'false bottom' for terrs w/ water features allowing the water to drain through the eggcrate whereupon it gets pumped back to the water feature.

So...unless you're planning to do all that...you shouldn't need the eggcrate. The bottom line is that there really are no rules w/ terrariums...you can do just about anything and it w/ work pretty well.

Super. So at this point I am looking at a layer of ReptiBark on the bottom, some sphagnum moss, and then what? Perhaps some potting soil?

Brenda

I think you could just put your bark straight in there and use it as the potting medium. Many orchids are grown like that, and even if it breaks down and looses it's excellent drainage, so what? Ferns and plants like that won't mind. I think sphag would break down faster, as well as staying more soggy. But what do I know? =)

It all depends on what you want to grow in it. If only orchids, then bark would be fine. If only terrestrials then you don't want to use the bark by itself. I think you should do a few different zones with one area (maybe in the middle w/ the most light) using only bark, another zone using bark plus peat/potting soil, maybe a section that is only sphagnum moss.