Oh, I misremembered... a long time ago STM used to use a surface shader instead of a vertex/fragment shader to render, and *that* used emission. (frag/vert just renders the colour directly)
I attempted to rewrite that, so please give this a shot: https://pastebin.com/Dy04z8F5
Let me know how this goes! The shader is pretty simple, so it might not do what you want, just yet.