How about the cement they sell for automotive weatherstripping. I believe that comes in black. Being elastic it should absorb any motion between humidity induced wood dimensional changes and the rigid ceramic. I recall it being a bit messy to work with though. Even a black acrylic latex caulk might suffice.
As one who also dabbles in ceramics, it sounds like a fun project. I may one of these days take a crack at a ceramic pinhole camera myself.