I can't reveal how I got this information, and I can't reveal the name of the person who's confirmed it, although I can say that this confirmation is from an Apple employee.
They've confirmed that there is no feature in iOS 6 that can be described by the phrase "inter-app audio", and go on to sympathize with the lack of support for multiple application audio in iOS.
This says to me that the whole inter-app audio thing isn't going to get in the way of what AudioBus are trying to achieve, mainly because it would appear that this isn't something Apple are looking to get involved with. Yet!
Let's see how this pans out, and a huge thank you to the person who informed me.