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.
This proof of concept demonstrates just how (relatively) easy it is to make your custom player.
At the time being, the crew on #awkwardtv is working on an improved version that can be released.