You can use VLC Player. The UI/UX is awful, but it gets the job done.
The Apple TV and your Mac (with the Keynote file) have to be in the same local network when setting up the Apple TV (Apple TV works w/o network after setup).
- Export the Keynote file as a movie (File > Export To > Movie…)
- Install VLC player to the Apple TV
- Switch to the "Remote Playback" menu tab in VLC
- Activate the "Enable Remote Playback" button
This enables an HTTP connection to the Apple TV via the network address(es) shown in the VLC Remote Play screen on the Apple TV (e.g. your Apple TV IP address or Bonjour address).
You can then open the shown IP or Bonjour address (http://…) in Safari on your Mac and upload the exported presentation movie to the Apple TV via the web interface (drag-and-drop or the "+" icon).
After uploading, the (presentation) movie file appears in the bottom area of the VLC Remote Playback screen and can be selected to play.
Use the playback options (swipe down on Apple TV remote) to enable playback looping.