David, your good work should be brought to the attention of Peter Gannon.
When the work is good enough, he'll hear it.

Actually, when the work is good enough, I'll start posting on the forum for feedback.
I've already written to Peter about this. I'd like to see something better than
Sinsy, but there aren't a lot of people promoting singing voice synthesis programs.
There are a lot of free projects (including
Sinsy, which recently released their code), but most of them are abandoned, and the websites are filled with dead links.
Vocaloid would be too expensive to add to BiaB.
On the Mac, there's a program called
VocalWriter that uses articulatory synthesis, but it's never been ported to the PC, and (unfortunately) the author died several years ago. His wife is looking to sell off the product, but I doubt PG wants to go that route.
The Mac actually has a quite nice vocal synthesis program built in, but there's no PC equivalent. (Finally, somewhere the Mac users are finally ahead!

)
I believe the best commercial bet is
SoftVoice, which has has a rather nice library. The owner is pretty busy right now, but he said he'd try to get me a demo. You've probably heard their product in some incarnation -
Software Automatic Mouth,
MacInTalk and
Talk-It! are some examples. The speech sounds a bit synthetic, but it's understandable and (to my ears) superior to
Sinsy.
If I get a demo from
SoftVoice, I'll post the results on the forum. But I haven't seen the API yet, and creating singing output isn't entirely trivial.
In the mean time, I've dusted off my own speech synthesis program (mainly because
Sinsy seems so deficient). It uses formant synthesis for vowels (like
SoftVoice, but it uses sampled consonants. It can now read the MusicXML file from BiaB, covert the words to phonemes, and output a .wav file.
Unfortunately, the new consonants broke the timing code, so I'm trying to fix things back up again. Once it works, I'll start posting some examples to get feedback.
It's written in Java, but once it's done if it's any good I'll give some thought to re-writing it into C (or with a language which can compile to C).