I doubt its a physical limitation, it does output ac3 on the optical out if you a watch a movie over the internal tuner, from usb or even when you stream it over ethernet.
Seems more like a software limitation.
It would be a truly awesome feature.
Not needing a hdmi reciever or a reciever with lots of digital inputs, just an active surround system with a built-in DD/DTS decoder with 1 optical in would be enough.
Not needing to switch inputs on the reciever additionally to switching sources on the TV.
Not needing to break your x360 component cable jack just so you can connect a hdmi cable and the component cable.
Damn, if it would work I would have bought a surround system model with the dd/dts decoder built in and donated my old reciever to better causes to save space and have less remote controls.