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I give you kr_renderOverscan 2.0! It'll render overscan perfectly for the times when your camera needs to line up with other cameras in other applications (such as Nuke).
Just select your camera, run the script and it'll pop up a dialogue box displaying your current resolution, target resolution and a percentage you would like to overscan. Through some nifty math, it'll adjust your render resolution and change your cameras postScale to give you pixel perfect overscan.
Since I use vray at work, I had to include 2 lines of script to change the resolution with vray. If you don't use vray, just delete the lines: