You might be able to use two programs together and get the effect that you are looking for.
1 being a text to speech engine... that will take care of that end of things.
the other piece of software that you could use would be a voice chat program called SeeStorm, it takes the voice, and animates a 3D head..
You can get SeeStorm
here.
By routing the text to speech output to the input of the SeeStorm voice chat program you should be able to get your desired effect.
good luck! and I hope that this is of some help
EDIT: SeeStorm is not full screen, but maybe you could run the resolution really low, and mask off everything that you dont want to see.