Hey Si

If you want to just render the shadows then have a look at rendering layers, it depends on the version of maya you have but you basically set up layers (like you would for geo but as a render layer) you then add the geo that you want for each layer, (say you were doing a car on a background you would make 2 layers one with the car and one with the background) then you can either (depending on your version of maya) use the flag options (thats in the render layers control) to set what you want to render, beauty, diffuse, colour etc etc for each layer.

I dont think that theres an actual way to just get the reflections without making a 100% reflective material and rendering it as a new layer and then comping this over the top.

Cheers

Steve