Hi Pozzi!
Im kind of in the same boat, but Im having problems with custom Geometry/Archives. It appears all nice and beautiful in the viewport but mental ray doesn't want to give me anything either.

I hope you have arrived to a solution and please let me know about it. For now, I can just leave you with some obvious things to check:
-Preview tab : 100 percentage in the preview settings
-Output setting: set to render and mental ray. Click autoset, so it cant cut any geometry.
-mentalray settings: rendermode should be live.

good luck!