This is going to sound super basic (and I'm thinking the answer is no after some googling around):
Is there anywhere you can download an iso for the android tv os? To use on a custom box?
I was thinking it could be pretty awesome to get a NUC or other similar mini-PC and run Android TV on that. I want to get a shield for the bedroom but my GF veto'd it based on the "gamer" look.
You can turn down the green light on the shield. Maybe even off. The shield is still what I would recommend. I love the controller on the thing and the remote is great too. Huge selling points on the remote and controller is the fact that they can be recharged with USB cable and have headphone jacks
http://www.android-x86.org/ this is the closest that I know of, it's android just not android tv
Seconded to /u/Dsk001 . Android TV is effectively just a custom launcher, the leanback launcher , beyond that it's just standard android under the hood, install the leanback apk and, you've got an android TV.
It would probably be easiest to stick with ARM-based devices that already run android (as opposed to a NUC/x86 device) for compatibility reasons. Easiest solution would be to get a nexus player but, if that's out as well, there are a lot of boxes that run base android out there that you could make work. The only difficult part is that you're going to probably want something that can run Android 6/Marshmallow, I don't think the leanback launcher is compatible with anything earlier.
Get a Nexus Player for the bedroom. Look into Pure Nexus if you want a third-party ROM.