Sorry but you'll need to go back and forth: out of Lightroom and into iPhoto. Setting up a workflow of flagging and using Smart albums can make this easier. But basically, those images need to get into iPhoto to work with Apple TV. Have you considered using Apple's Aperture? Aperture integrates nicely with Apple TV. That's what I use. However, be warned that Aperture is a resource hog and requires more of the higher end processes to work really well, especially if you have large photo libraries. You'll want 8GB of RAM and a newer graphics card (basically a newer Mac) and higher end CPU. The minimum requirements that Apple says Aperture needs are minimal at best.
Have you thought about using iPhoto for organising your photos (and working with the Apple TV), but having Lightroom edit your photos through iPhoto. i.e. when you choose edit in iPhoto it opens the photo in Lightroom.
IPhoto preferences > advanced > edit photos and select application and choose Lightroom.
HI, just in case you are watching this thread or in case someone else comes across it: if you set up a Publish folder in Lightroom to go into iPhoto, it will be synced as you add, remove or edit photos. It's not completely automatic, but it does take a lot of the manual moving of photos out of the equation. Plus the photos will all be saved as jpegs at whatever resolution you want as they are Published to the specified folder.