It is a bit more complicated. Depending on the material applied to your mesh we have to render it in specific ways to make sure it receives lighting, casts shadows, is sorted properly with translucency if it is not opaque, etc.
That is different from Canvas which allows you to easily draw 2D or 3D shapes over top of the main scene being rendered.
We have a sample of how to do this from a plugin internally, hopefully we can get that out to Rocket users sooner rather than later.