Not sure I understand your question. Mental Ray is nothing but a renderer. It can run within a program like 3ds Max, which is what I use, or it can run as a standalone application for batch processing or on a separate computer for use as a render farm.
Well, in Maya, Mental Ray is provided in a fully-integrated fashion... I have no "stand-alone" capacity. And, again, in Maya, I have the ability to create complex "material" networks, but I know of no way I can make use of any "built into Mental Ray" materials which are independent of the Maya type materials.
Doesn't mean it's not there, just that I haven't discovered it. Then again, these programs are REALLY deep (and I think Maya is moreso than some others), and I'm doing this as a hobbyist, not professionally (though I strongly believe that what I'm learning "as a hobbyist" now will end up serving me very well in the future, as the line between "engineering software" and "rendering software" continues to fade away).
As of 3ds Max 2009, there are two main groups of materials designed for use specifically with Mental Ray: The Architecture and Design library, and the Pro Materials library. Both provide a variety of optimizations and unique features via Mental Ray, but they will not work at all with the regular scanline renderer.
As far as I know, the Mental Ray renderer for Maya is identical to the one for Max, though the interface and the way some of the features are implemented may be somewhat different.
Exactly what I was getting at.
I have not found anything in Maya which renders only in one renderer... well, except for "hardware particles" of course, but I can set those up to emit software elements in turn, and get equivalent, or even better, effects than I'd get with the hardware ones anyway (though at, as always, a big performance hit!).
So, I suspect that what we're looking at there is an issue with how the renderers are hooked into the software. I suspect that if I had the stand-alone Mental Ray version, I'd have some way of using that same library... but as far as I can determine, this doesn't exist in the "integrated with Maya" version.
Then again, if someone knows better... please correct me!