My chickens would roost on it in a flash, and poo all into the wall cavity; I've fought with them about this in two very comparable situations. OTOH maybe other chickens are different, who knows.
If closing off the gaps (which I sure would), I'd use something solid, not chickenwire, so that feathers and bedding and dust don't accumulate down there at the bottom of the wall cavity where you can't get to them. Pieces of 2x4 would work well, or even just flaps made from scrap linoleum. I'd set them at an angle to prevent roosting.
I've been in horse barns when stall boards that enclosed a similar open-topped wall cavity were removed after years... there can be literally FEET of dust and shavings built up back there, plus the occasional mouse/bird/barncat skeleton
If you are ok with the open wall cavity, just don't want roosting, I'd suggest stringing a fairly strong wire (like steel electric-fence wire) tightly 4-6" above the top of the half wall. It will need to be *strongly* affixed to the wall with good long screws, and well tensioned, but when a chicken tries to land up there the wire won't let it perch ont he plywood and the thinness of the wire itself will keep chickens off it.
Pat
