Options seem to be very limited! I believe Macs have built in software that does allow some kind of TTS manipulation, but I don't have a Mac. There's Vocaloid, which now has an English version (doesn't work so well as Japanese unfortunately) but apart from those two, not much else that I know of.
It's frustrating, I've searched Google for over an hour looking for software that would allow me to play with TTS voices, but I'm finding nothing of use. I don't need the voices to sing in the true sense of the word, I just want to change the rhythm/tempo.