J:
What about the Electro Harmonix Micro Synth pedals? Or what about a 'vocoder' modulated with the instrument part instead of a voice (which is I think very similar to what the Electro Harmonix stuff does)?
Whoever mentioned Eventide, there are some patches in the Eclipse (and probably their other units) that will do this, but they take a lot of tweaking to get right... Ring modulators, if set just so, will do this very well. Never used their plugs, personally.
Best, Marcel