mental ray doesnt support it?
I dont know mate, rendring to an avi from maya is a bad idea anyway. I always go out as tiffs and then bring them back together. Why I hear you ask: Because lets say you render it out and your compression is crappy and you'd like to up it a bit.... if you have rendered as a movie you're screwed. Because you'd have to re-render it again. If you do it as tiffs then all you have to do is launch premier and fire it back out again at a different compression. There is also the argument that you end up with enough artifacts in your renders as is, why introduce more at the rendering stage by using compression?
Hope that answers your question.
Technical Director - Framestore
Currently working on: Your Highness