Could Media Converter take every 5th frame from a scanned movie for the first 40 seconds, crop and scale it to an icon as a .gif or .apng?
I've done some experiments with a similar photoshop batch script and had great results.
Animated icons, or preview clips when hovered over the icon would be an awesome addition. It requires a little scripting to get apng working in xbmc, but gifs work fine.
Many skin creators have asked about similar functionality, but the xbox doesn't really have the raw power to do this (it would take forever) and the process is different on PC, Mac and Linux. A 3rd party app that could generate such content would be very helpful. Given the nature of this program, it seems a natural fit. Also, many movies simply don't exist on IMDB or other sources. This could create similar icons for them and keep the look consistent.