Just noticed that I didn't answer this. There must have been a similar post somewhere.
For framebuffers to work, you need them turned on in both the options block and the camera block. Is your output shader question really a question about how to enable them in the camera block?
I'm not sure how to do this within Maya, without using custom text, writing an mi file, and rendering with standalone. However, I know it is possible, since there are several folks who have written multiple framebuffer shaders.
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