Unfortunately unreal doesn’t really have separate user render passes out of the box.
Doesn’t each material domain correspond to a separate render pass? I used a postprocessing material and it does render the borders in the custom depth pass on the top of the province meshes, but the pixels are “shaking”, there is a blur effect even when still.
I have 20.000 separate border meshes to render on the top of province meshes in the cheapest possible way. Decals are probably out of the question for lack of scalability. I’ll look into the runtime render texture solution you mention though.