I have never seen plaster and wood lathe butted directly to a brick wall without some sort of vertical structural member. I'm not saying it doesn't exist, just that I've never seen it and can't think how it would even work. Usually, the wood lathe runs horizontally from one 2x4 (or some other sized wood structural member) to the next, the lathe piece is nailed at both ends with one nail, and the plaster is applied over lathe. Even in the old brick apartment building I lived in, this was the case. Are you sure there are no vertical 2x4's in the wall? If there are then just screw directly into the 2X4's. Even if, for some reason, there are no vertical structural members, a screw directly into the plaster/lathe is pretty damn strong.