http://soundlab.cs.princeton.edu/software/rt_lpc/ is a small real-time LPC analysis/synthesis application and visualization. Currently, it allows pitch transformations (of the glottal pulses) and it is also possible to "sing" with it using a MIDI controller for the pitch. It also comes with a library in C that performs Nth order LPC analysis and synthesis. Transformations, such as formant shifting, can be added in the middle (e.g. moving the poles of the LPC filter). this library/app and LPC probably doesn't do everything you want, but may be a good starting point. Best, Ge!