I guess you could try unplugging the output from the centre or front l/r speakers and plugging your headset in instead. How well that would work is entirely dependent on how the sound is distributed amongst the 7.1 channels. The centre is also shared with the sub, so even if it works the effect might be a bit weird.
It's something I've wondered about since first using Ventrilo with LOMAC for multiplayer, where I'd use separate audio devices for the game and comms (onboard sound and a soundcard, although like HHF says, a USB headset also works, as would one something as simple as this
). I've no idea whether it's possible for a single app to use two audio devices, but it would be a fantastic feature.