ericIII from the AwkwardTV crew has created a plugin for the AppleTV that, like Sabertooth’s plugin, hooks into the main ATV GUI. However, this one plays videos, and sports a (for now, hardcoded) list of .AVI files that are not located in the Media Library.
To install, just copy the plugin to /System/Library/CoreServices/Finder.app/Contents/PlugIns on your AppleTV.
After it's in place on the AppleTV, you need to restart Finder (either with killall Finder if you have killall installed, otherwise use ps awx|grep Finder to get the PID and then kill PID.)
ApplianceTest - source to the plugin used in the video above. See README.txt. Be warned, however, it's absolutely ugly.
FRAppliance 101 - a much better introduction to writing plugins.