That's a mouthful.
On a job recently I got some secondhand Nuke goodness when a lighting TD showed me how they were using UV passes from Maya to "auto-track" some footage into their shots (I wasn't on that job). The basic issue was that a lot of footage had to go onto screens in the spot and some of that footage wasn't ready or the client was changing their mind about what the footage should be. So, in short, any manual trick to the get the footage locked onto the CG screens required redoing some additional work in post, in terms of tracking, etc. Additionally, sometimes late notes were coming in about animation changes to the screens themselves, which meant that there was no option to render "in-camera" in Maya, it had be done in post.
So the simple solution was to render the screens with properly adjusted UV's as UV passes. This automatically locked the footage in place and wasn't dependent on any additional work once the comp was built, even if the footage had to be completely replaced or the animation of the screen itself changed. Pretty sweet. While I'm sure some people think this is obvious, I didn't (and neither did a lot of the other people I was working with. So I'm not the only ignorant one.)
In any event, I also realized that I never really followed up on any render pass stuff in Maya 2010/11, so the first vid below is basically what we're trying to do and how to get custom color passes out of Mental Ray (as opposed to the "regular" passes that are pre-set, like reflection, diffuse, etc). I use this method to get out RGB matte passes and a UV pass in EXR format.
The second video is how to actually use the UV pass in Nuke to get the basic effect we're looking for. Nothing crazy, just basic use of the UV pass.
Maya/Mental Ray: Custom Color Passes (2010) Part 1 - UV Pass from zeth willie on Vimeo.
Maya/Mental Ray: Custom Color Passes (2010) Part 2 - UV Pass from zeth willie on Vimeo.