I would like to the Plex Installer on the AppleTV (Black) it would be a nice option alone with the other installers like Boxee etc…
Yes, however doing so would be to pirate the Plex iOS application from it's developer.
The mechanism currently exists to install the Plex iOS application via iTunes by modifying the UIDeviceFamily, however in it's current state, you do not have the ability to effectively use it as it does not support hardware keyboard inputs.
If Plex were to port a version of their Application to run under the AppleTV2,1 and elect to distribute it outside of the App Store, than Firecore would be capable of presenting it within a launcher. But that also comes down to whether Firecore will be releasing a launcher for the AppleTV2,1.