on the fourth page of the WIP FubaR spelled out a nice step-by-step on how to render occlusion without using the render layers or occlusion presets, which isn't really hard as long as you know how to connect nodes in the hyper shade (right click & hold on the lower right arrow of the mib_amb_occlusion node, go down to outvalue, then outvalue (not outvalueR, G, or B)... it'll create a line from the node to wherever your cursor is. from there, left click on the surface shader, click other, and the relationship editor should come up... on the left (the mib_amb_occlusion node) click on the word outvalue - not the plus next to it - and then on the right click the word outcolor.)
after that, you should do as he says and increase your samples... after that, all you need to do is highlight all the objects you want to render in clay, apply the surface shader, and then render with mental ray (i recommend using the production preset) and you should get the ambient occlusion look, like so (i only applied the surface shader to the sphere and the plane)
at the same time, if you don't want to do all that, you could just render in mental ray with final gather on (turn on final gather, then increase your FG rays to anywhere from 1000 to 10000 - in this render i used 2000), and then in the viewport from which you want to render, click view, camera attribute editor. scroll down, and under environment, change the color from black to pretty much anything (your render will show up on a scale with this color, so a lighter one is prefered, but actual hue doesn't matter so much), and you'll get something like this...
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