What about using a tablet to direct the piece by drawing? You can get values for the intensity, speed and volume (up/down) pretty easily and intuitively. Even

What about using a tablet to direct the piece by drawing? You can get values for the intensity, speed and volume (up/down) pretty easily and intuitively. Even better if its linked to the voice generation system in real time, then you can save/redo sentences etc. as you go along.