Hi n88tr,

here is a quick rundown of how to do this.

In mental ray settings, on the "features" tab, scroll down to "countours", check "enable contour rendering", then scroll down to "draw by property difference" and check "around all poly faces".

Assign different materials to each object. For the one you want to render with a wireframe, go to it's shading group tab, scroll down to "contours", and check enable contour rendering. You can choose the wire colour and width here too. To smooth out the wire, you can go back to the mental ray settings and adjust "over-sample" under "contours".

Very quick test I done here used final gathering, white environment and no lights.

Hope that helps,

cheers

gubar