Not sure if this method has been mentioned before or if a similar method has been mentioned before. For the past two days I have been trying to render out a reflection pass out of Maya; my scene is a ceramic bowl on top of a reflected ground plane.
Unfortunately you can't just "click" the reflection pass button because there is none.
When starting to figure this out I noticed that the bowl's reflection was showing up in the diffuse pass and not in the spec pass as I had hoped, which meant that I needed to isolate that reflection into its own diffuse pas. So I started to break down the scene into layer passes. Reflection was turned on on the color ground plain shader like it should; however, the reflection was also showing up with the diffuse of the ground plain, which meant that the reflection did not have its own alpha. So what I did was I duplicated the ground plain object and applied the "Use Background" shader to it; I then turned the reflection and specularity all the way to 1. I also turned OFF the reflection entireley on the color ground plain shader because the bowl's reflection would be handeled by the "use background" shader plane object.
Now I started to create the render layers. The bowl, color ground plain, and "use background" shader ground plain all had their own render layers. The result was very close to what I wanted. The bowl rendered out fine; the ground rendered out fine; the "use background" shader ground plane also rendered fine meaning that its diffuse pass showed only the reflection from the bowl. Unfortunately there was one issue with this diffuse pass, and that is its alpha. The reflection's alpha showed up as the right shape of the reflection, but it was a solid white alpha which meant that the layering in post would not be correct.
So I looked at the image's rgb channels to see if one of those looked like a good candidate for an alpha. Fortunately the red channel looked like a good match. In shake I did a quick reorder and made the red channel the new alpha. When I layered everything together, the reflection worked perfectly after some small fading and color correction.
So as a recap:
1: duplicated the ground plain that showed the reflection and applied a "use background" shader with the reflection and specularity to 1.
2: Turned off reflectivity on the color ground plane shader
3: Put each object into its own render layer and rendered
4: substituted the reflection's diffuse pass's alpha with the red channel and composited.
I hope this helps!