My initial reaction to this idea was that it was totally wrong. I was even about to reply saying how bad of an idea it was, but now that I think about it a little more, I think Benanti could be good in the role. We know she's capable of dramatic acting, not just her usual goofiness. As for the singing, she doesn't have the typical Diana voice, but it could still be great. I'd love to hear a rendition of I Miss The Mountains in her lovely mix.
I feel like any commonly-belting actress who is over 30 is at some point dreamcast in the role of Diana. I like the idea of Benanti, someone who doesn't typically belt, taking a crack at the role.