Probably never. Apple is a closed playground.
Only on Windows though, isn't it?
In Mavericks, AirPlay receivers show up as separate devices in the sound preferences. (though not in Media Center - only a generic "AirPlay" device which picks the first one on the list)
AirTunes was reverse-engineered and open-sourced on Windows almost a decade ago at this point.
Because this is implemented in hardware (the original AirPort Express) Apple has not, and will not be changing it.
The lack of proper AirPlay support is probably Media Center's biggest omission when it comes to streaming audio to wireless devices.
There's nothing I know of which is as cheap and reliable as the original AirPort Express for streaming lossless CD-quality audio to a DAC in a one-box solution.
I wish you would reconsider your stance, after being "burned" on developing iPod support when they changed operating systems.