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.
http://ericiii.net/sa/appletv/ApplianceTest.zip is the source to the plugin used in that video. See README.txt. Be warned, however, it's absolutely ugly.
The file browser/player has been released, see ATVFiles.
FRAppliance 101, an introduction to writing plugins.