There are a lot of very useful shader hidden in MentalRay such as Rayswitcher, Matte/shadow, Mirror/gray ball etc... but they are for the moment not supported by autodesk.
How to see those shaders in the Hypershade ?
First go in the folder : Program Files\Autodesk\Maya2012\scripts\others and find the mentalrayCustomNodeClass.mel and open it with a text editor.
At the end of the file you will find this line " $enableMAPShaders == 0 ) || " change the "0" to "1" save the file, and restart Maya !
If Node are not displayed in the hypershader you can type in the "mel" input the line : createNode + the name of the node for instance :
