Factorio Forums

I would expect it to be drawn under the tiles, but isn't.

There are several renderers (EntityRenderer, BuildingRenderer, TileRenderer, RoboportInfoRenderer, DebugInfoRenderer, ChartRenderer...), and they for the most part operate on separate ranges of layers. LuaRendering operates within EntityRenderer only.

You can make a modding feature request explaning what exactly are you trying to achieve, I'll see if I can modify the systems to make it possible.

As far as i know tiles do not support (partial) transparency, so anything "under" them would never be visible? That sounds like the bigger problem here.