A plasma cuts at very high concentrated temps and if you cut from the back I believe it would work. Old porcelain is so thick I'm not sure the plasma would get ground from the front without some kind of starter hole to reveal the bare metal edge. Being the glass would be melted, I would expect a smooth edge but my concern would be warping especially on long cuts. Unless someone experiments on an old sign and gives us some news either way, I can only speculate based on welding experience.
Looking for old, rare, auto light bulb tins