It seems there’s no equivalent to Blender’s “Sheep it” and “BURP” collaborative render farms but for Mental ray or any other renderer.
But maybe there could be a more simple system, without additional software. Of course it would be entirely based on trust and would probably not be used for commercial purposes, but just a website where anyone can share his projects to be rendered. Would it be too dangerous or illegal ? I think there are many people who wouldn't mind sharing some personal projects to win some render time.
Once you've tried grid computing (like with Sheep it), it becomes obvious that everything should work like that, and it's quite frustrating to see that very fiew things do.
edit :
What is this [b][asm-mentalray](http://sourceforge.net/projects/asm-mentalray/)[/b] that claims to use grid computing with BOINC ?
Looks like an unfinished project.