I'm going to assume this works pretty much the same as the HDAV 1.3, as everything I've read so far indicates so.
The way that seems to conform to what most users are seeing is this. I also tested for this scenario and it does look like it's the right one. There are 3 devices actually, but only two (speakers and digital device) are visible by Windows and other programs that are not TMT.
The "digital device" is S/PDIF and will behave as any other S/PDIF device. The "speakers" device acts like a regular analog device, and indeed if you're using the HDAV 1.3 deluxe, it will control the analog part. The third device, the actual HDMI one, grabs the digital stream from both devices (before the DAC with the "analog" one) and is controllable only by the Xonar CP. Since the "speakers" option behaves like an analog device, if you select that as default in Windows, you lose the ability to bitstream anything, but only with players that don't let you choose S/PDIF output in their options, like sometimes happens with Media Center.
Asus TMT and TMT3 have the ability to access the HDMI device directly, thus when they do, it doesn't matter which device you have as default in Windows.
Wasapi exclusive in foobar2000 and others will still work, but since it can't see the HDMI device, it will have to work with either of the 2 devices it does see. If you have it work with the "speakers" device, it won't auto switch because multichannel LCPM is not passed untouched through the HDMI device and it's at the mercy of the Xonar CP. It will still work by bypassing the Vista mixer. You can test this by changing the sampling rates in the Vista playback device options to something obvious like 16000 Hz. It won't have an effect if you're using WASAPI exclusive with that device. Or just try to play something else at the same time, it probably won't work since it's being used exclusively by your first program with WASAPI-exclusive mode.
If you have WASAPI-exclusive use the "digital device", you'll only get S/PDIF sound, so 2-channels of 16/48 is the max you'd get. Auto switching will work because S/PDIF sound is passed through untouched through the HDMI device. That's also why you can do DD/DTS bitstream with this.