That sounds rather complicated... I agree the wise approach would be a driver, since it has to catch the audio before it is delivered to the sound card.
The problem is that:
1st - there are bound to be incompatibilities between programs and the driver, as there are with any driver, but this is not such a big deal
2nd - true 5.1 source detection doesn't seem so easy, since there are several ways to communicate 5.1 sources to the card (DirectSound3D, A3D, EAX, OpenAL, etc.) and it seems highly dependent on the sound card
3rd - general system slowdown, as every bit of sound has to be processed
I believe this would be a full time job...
Maybe JRiver are willing to do it, but it's hard, and it would be a lot simpler if the sound card manufacturers just included it in their drivers. Their drivers already do all that except for generating the central channel's sound. And maybe the option is there, only there is some weird procedure to turn it on...
You should ask Creative for support on this matter, maybe they can help...