Your idea for a Rift app sounds like an excellent idea! It'd be a lot of fun to try to create a realistic model of an audience. The small motions would make all the difference: that guy who suddenly looks up and stares at you, the person who sits stiffly with their arms crossed, the person who just can't sit still.

However, one big problem is that, currently, the Rift is designed to be used when sitting down. What you really want is some way to track your position and stance within a fixed area (the 'stage'). Then you'd be able to practice your whole stage presence, not just your spoken deliver. I'd love a program which could analyse your movements, and tell you that you look down too much, or that you're slouching, or that your hands shouldn't just be hanging limply.