You didn't look close enough, but to do that, you have to know where to look!
There are only two books that deal with this subject. The first is the original Rimsky where in the book he covers orchestration for vocalists. Then Charles Koechlin does the same, but it's all in classical French.
I think the question you posed is a good one, but not many here know how to answer it with the exception of Daryl (DG).
If you wanted to look at an accessible opera score to try to answer this question, look at Bizet's Carmen with the complete score available from Dover.
I'll check with some of my orchestrator friends to see if they know the answer. Most are really busy so it may take a while to get the answer.
One way to spot check might be to get the Phantom Menace score and compare the orchestration to the movie dialog.
Another consideration is getting Leonard Bernstein's score to West Side Story which omits the Violas altogether and compare from there.
This is really a great question,Mathis. I'm sorry I don't have the wisdom to give you the right answer off the bat.