You know, FESTIVAL (the popular and quite good
opensource speech synthesis
package) has a mode where you can feed it data to sing.
It could probably not be very hard to write a similar GUI where
you edit a pianoroll to make festival sing. 
 There is a much better speech synthesizer, known as DECtalk which can sing
 quite well, and it is very easy to programme. The nicer thing is, it is
 available for Linux as well! I have it here, so I will did up a text file,
 and post the resulting OGG if anybody is interested :)