If you have an Apple TV 2 running the latest firmware—iOS 7.1.2—then you'll be happy to hear that you can now jailbreak using FireCore's well-regarded Seas0npass tool. Unfortunately this jailbreak is of the tethered variety, but outside of the initial hassle, that doesn't matter much for something like a set top box that's almost always powered on.
FireCore quietly released a beta update to Seas0npass, which comes in at version 0.9.7. This update works with OS X El Capitan and Apple TV 2 firmware 6.2.1 (iOS 7.1.2).