I wish to light a model without it showing highlights or shadows, so it looks 2D and blends in perfectly with a background image for a layer composite. The default Houdini light adds shadow, so the best option seems to be the Environment Light that lights the model evenly from all sides, with an Intensity of 1 and pure white colour applied for the light, and turn off reflectivity/metallic etc on the material. Is this the best option, or is there some way to make the material (Principled Shader) or the model ignore "3D shading"?

For no lighting, use a constant shader (emissive). This will skip any light and shadow calculations, letting the texture set the on screen color directly.

Thanks jsmack, that's brilliant!