If you just need the outline the easiest way is to use maya vector.
If you go to the maya vector tab in you render globals there should be a box called "edge options" Simply check "include edges" and there you go. Depending on the look you want you might akso want to play with the "fill options"
I hope this helped. Before you ask I don't know how to do this in mental ray, but if you want to know,

https://www.fenriz.org/projects/maya/

that explains some more detail

good luck