CamTwist lets me pipe custom content (such as images or video) to that webcamera input, and SoundSiphon makes it easy to have system audio interpreted as microphone input.
Q. Is it possible to achieve something similar to the above, but on the iPhone?
E.g. I'd like to be able to use a native iPhone app (such as Skype) and pipe custom video to it (rather than using the in-built camera), whilst simultaneously broadcasting a custom audio file (rather than the in-built microphone)