Guy, I have to admit in a not-permaculturally-enough moment I bought those flat pieces I have on top of the bench.
Mike, so, I agonized over the thought for a while, and got lots of feedback online. Most of that said "it won't work." I didn't care much, since I was experimenting, and still am. What I discovered over the summer was that while I was using the system, it could get wet and then dry out again when I used it with no ill effects. The rock covering made it so we weren't sitting in mud when it was wet, and for other spots I'd just throw a piece of scrap wood over it when I wasn't using it.
Fast forward to now. This system hasn't been run since October, and we get about 30" of rain annually, mostly between late October and May. So, it's seen a LOT of rain. Guess what? It's fine. Like, totally, completely fine. It's wet, but I'm pretty sure I'll just dry it right back out again once I start using it. I'm going to do the rocks over the whole thing sometime this spring. I think it looks nice and it does make the bench more functional since it's not muddy when wet. And they dry quickly once it's lit. So yeah, no problem. Even the hollow chamber held up with those little sticks is totally fine, which I expected to be a mud pile at this point. Keep in mind I'm referring only to a system with an internal heat source. I wouldn't expect a cob monolith bench with no internal heating to ever dry.