I believe this is possible, but I haven't tried this. Basically you want to go into your iTunes preferences and to Advanced. From there I think you'll have to uncheck the "Copy files to iTunes Music Folder when adding to library" This should allow you to keep your files on another volume and then drag the file into iTunes. Instead of copying the file to your MBA (or wherever your iTunes library is), it will just create a link to the file. Choose to sync that file with your Apple TV and hopefully you're good to go.
Let me know if that works because I'm curious myself.
The downside to this method, which should otherwise be completely viable, is that your network may not be able to handle it, especially if it's wireless. Because the
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TV draws its files from iTunes on your computer, which will then draw the files from your Time Capsule, the files bounce around a lot on your network.
They go like this: from the Time Capsule (as your network storage) to your computer (where iTunes is running), back to the Time Capsule (as your wireless hub), then to the
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TV. This may seem crazy, but it has been confirmed that the files will not go directly from the Time Capsule to the
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TV. The Time Capsule will have to wirelessly transmit, receive, and re-transmit every bit of data. This has caused some people's systems not to show movies smoothly, with periodic stuttering of the video.
It sounds like it has worked ok for other people, though, possibly those with an 802.11n network and good signal strengths with low interference. You might also be able to reduce the likelihood of network congestion by connecting your
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TV to your Time Capsule with an ethernet cable.
Let us know what kind of results you get.