I'd imagine that Sling Inc. would love to do this, and also I'd imagine the choice of whether it would ever happen is up to Apple. I'd lean toward "it's not gonna happen" though, and AirPlay is the best we're going to get unless Apple opens the Apple TV to run apps via an App Store (much like the iPhone was opened via the SDK/App Store).
One thing to ponder--let's say Apple invites Sling Inc. to create an Apple TV app/channel. How do you simulate a remote with dozens of buttons with the OEM Apple TV remote?
Simple enough... in the Sling world they provide a graphic of the remote control of whatever device you are controlling... ie: in my case DirecTV remote.
Now this works fine with my Retina display MBP, but my wife has the earlier MBP which does not support AirPlay. Also I have found that if you use an iPad and AirPlay to send my Sling image to my TV the image quality (resolution) is much worse from the iPad than it is from my MBP. This may be due to the fact that my iPad is an iPad2 which has lower built in resolution than my MBP... but it is painfully obvious when putting an HD football game up on my HD screen.
Now as to how to control this with the AppleTV remote, you would simply use the left right up down navigation on the Apple remote to move the cursor around the graphic remote supplied by Sling. Not as elegant as a mousepad for moving the cursor but definately workable and this way I do not have to run an extention cord to keep my MBP powered for whatever period of time I want to watch TV... also I would wind up having to buy a new AirPlay MBP for my wife and set up extention cords for her... which leaves us both prone to tripping over cords and doing horrible damage. Much simplier to simple have a Sling Client on the Apple TV devices attached to the displays in our townhouse so we can watch our programs when away from the main house.
"One thing to ponder--let's say Apple invites Sling Inc. to create an Apple TV app/channel. How do you simulate a remote with dozens of buttons with the OEM Apple TV remote?"
The same way current connected devices use the full remote interface requiring only the four arrows and a enter / ok button to move around the U.I on the screen to access all the functions. Boxee, Google tv hardware and the wdtv boxes all use the same U.I and don't require a virtual remote to function. Now, is it the ideal way to access your remote control functions....no, but it is usable and I don't see why sling would change the interface for Apple TV.