While you can root and flash custom ROMs to your mobile devices liek smartphones and tablets, some people may think that's where the possibilities stop. That isn't the case however as you can basically root and ROM nearly anything running a version of the Android operating system. Take the Android TV devices for instance, more specifically the ADT-1 or better known as the original Android TV box which Google displayed and showed off back at Google IO 2014 last year. While it's still completely functional and anyone that has one surely still uses it, there's no denying that current Android TV offerings are further along than the ADT-1.
One way this is apparent is with the software. The ADT-1 was updated back in February to Android 5.0.2 to keep it current for the time, but once Google released the Android M developer preview, the Nexus Player soon received an available compatible build of it from Google to push it further up the software ladder than the ADT-1. While Google's first Android TV may not end up getting an Android M build officially, that won't stop individuals from trying to place it on the device, and now it's possible with the help from Team Win Recovery Project who has made their TWRP custom recovery compatible with the ADT-1 Android TV box. If you still have one of these devices you can head over to the TWRP web page to get the rundown on the steps involved and complete the process.
If you feel inclined to follow through with the setup to flash TWRP to your ADT-1 device, you'll be loading on the latest version of the custom recovery which is version 2.8.4. TWRP has made it possible to flash the custom recovery using a couple of different methods, so whichever one you feel suits you best is the method you should employ. Flashing TWRP to your ADT-1 can be done either through the use of an app, or by connecting it to your desktop and flashing it using the fastboot method. In either case, doing this can set you up to place custom ROMs on the device once they pop up, and eventually that could mean ROMs based off of the Android M dev preview.