I've ported Movian (https://movian.tv) to Apple TV (although not released officially yet). It's not really ready for submission to Apple yet so I've not bothered with that just yet.
It's capable of playing h264 (using HW-acceleration) and also MPEG-2 (including GPU assisted de-interlacing) from Tvheadend.
If anyone is interested in testing I can provide ad-hoc builds (this requires that you install via XCode from a Mac to your AppleTV, ie, you need a USB-C cable), contact me at andreas@lonelycoder.com with your AppleTV UUID and be prepared to do some testing (I don't want to waste UUIDs since they are capped)
To get a feel for how Movian works/looks you can install it as a Chrome Web App from "this link" (Unfortunately there is a bug the version on the Chrome Web App store that causes interlaced video to be incorrectly stretched, but it works OK on Apple TV)