Honestly, I'm not buying that -- and I'm going to tell you why. Many years ago Intel did the same thing with their Sigmatel drivers they gave Dell. They kept saying it couldn't be done. After massive protest people found a way to hack the registry to achieve the functionality back.
Then, miraculously, Dell released drivers to make it happen. To sit there and tell me that the CMI8888 chip present in the Phoebus cannot do this function is absurd, because the STX which has the CMI-8788 chip can do it. Do you mean to tell me, that in one revision of the main audio cpu that they said nope, can't do it. Not gonna buy that one. Realtek does it, Creative Labs does it, IDT does it.
Another example is when the XFi-Titanium was released by Creative Labs back in 2008. The stock drivers did not come with the functionality for it. I returned the card. After I posted I returned it because the feature was not present, within 2 months drivers were released to bring it back. Again, it cannot be as difficult as you make it appear to be. Lastly, is the presence of the feature listed within the .inf. Again, if the functionality does not exist, then why bother writing it into the .inf file? Hence this taken from the latest .inf file for the Phoebus.
DriverVer=07/22/2013,7.0.1.30
HKLM,SYSTEM\CurrentControlSet\Control\MediaCategories\%HDAGuidStereoMixPin%,Name,,%HDAStereoMixPinName%
HKLM,SYSTEM\CurrentControlSet\Control\MediaCategories\%HDAGuidStereoMixPin%,Display,1,00,00,00,00
; Stereo mix interfaces.
;
AddInterface=%KSCATEGORY_AUDIO%,%KSNAME_eStereoMixWave%,HdAudModel.StereoMixWave
AddInterface=%KSCATEGORY_CAPTURE%,%KSNAME_eStereoMixWave%,HdAudModel.StereoMixWave
AddInterface=%KSCATEGORY_REALTIME%,%KSNAME_eStereoMixWave%,HdAudModel.StereoMixWave
AddInterface=%KSCATEGORY_AUDIO%,%KSNAME_eStereoMixTopo%,HdAudModel.StereoMixTopo
AddInterface=%KSCATEGORY_TOPOLOGY%,%KSNAME_eStereoMixTopo%,HdAudModel.StereoMixTopo
; Stereo mix Model and AddReg.
;
[HdAudModel.StereoMixWave]
AddReg = HdAudModel.StereoMixWave.AddReg
[HdAudModel.StereoMixWave.AddReg]
HKR,,FriendlyName,,%StereoMixWaveDeviceName%
HKR,,CLSID,,%Proxy.CLSID%
[HdAudModel.StereoMixTopo]
AddReg = HdAudModel.StereoMixTopo.AddReg
[HdAudModel.StereoMixTopo.AddReg]
HKR,,FriendlyName,,%StereoMixTopoDeviceName%
HKR,,CLSID,,%Proxy.CLSID%
StereoMixWaveDeviceName = "HD Audio Stereo mix"
StereoMixTopoDeviceName = "HD Audio Stereo mix mixer"
HDAStereoMixPinName = "Stereo Mix"
HDAGuidStereoMixPin = "{8c5716e6-c984-492e-a2fb-1385f2d0f3c9}"
So again my point is, if you want to omit functionality, why write it into the .inf file? Why add registry keys for the feature?
I'm sure there's one or two lines missing or a simple digit that will "turn it on". Sorry, but you have to come better than that for an answer.