Yeah, it works in a simple setup where I have the ramp position keyed directly. I suspect that mr isn't recognizing that a value is changing, since it's being driven by a switch node. I'll create another simple case where the ramp has a shader switch.

It's interesting, I found this in the docs:
"Animations

When rendering animation files or previewing animations inside Maya, mental ray for Maya exploits incremental changes. This means it determines if scene elements actually have changed between frames and just processes those changes, thus accelerating the render cycle. The check is based on Maya information and includes both geometry (plus instances) and shading nodes. If no changes are detected, the corresponding object or shader is not updated in subsequent frames and appear animated.
Incremental changes are not used if animations are exported as file per frame."

That sounds like this is the case, but I'm rendering out as individual frames, so it shouldn't be using incremental changes.