The Nexus Player is (so far) the only consumer device available that runs Android TV, which means a considerable portion of the people who own one are serious Android fans. It follows that they're prime candidates for ROM flashing (not to mention Android Police readership), so they'll be happy to know that they can now install CyanogenMod on their set-top box. CM 12.1 (based on Android 5.1) is now available in nightly form for the Nexus Player.
A custom ROM for a set-top box makes a little less sense than it does for a phone; Android TV is intended to be a rather encapsulated media-focused experience, with limited expansion via apps only. Unlocking the bootloader on the Nexus Player and installing a custom recovery to flash CyanogenMod is a little trickier too, thanks to the gadget's AC power supply. I'm a big fan of CyanogenMod, but without a Nexus Player of my own, I can't say what's different between Google's stock software and the custom ROM. At the moment there's only one nightly build available for "fugu," the codename for the hardware.
Remember that nightly builds are presented with no guarantees, and with Android TV being a new platform (at least technically) for the ROM team, there are bound to be a few teething troubles. If you're ready to go anyway, it's available to download on the CM database. It may or may not get a more stable "M" release during the next cycle.
- CyanogenMod Download