No, post-fader metering is not available (yet?). But sounds like you already know the root issues - mixing from the stage and using unmatched mics.
All of our backup vocals are on the same mic models, so that helps, but the lead vocalist is NOT matched. So many times when I am mixing from the stage, even though, it appears to me like it should be right, it likely isn't.
What I have taken to doing is setting one monitor mix up exactly like the mains for vocals, and that is my my vocal mix in my face, so I can hear what they hear out front. Life is easier when my wife comes to the gigs as she is our unpaid and under appreciate sound tech =)
EDIT: And it would AWESOME if there was a button that was a meter bridge that visually showed you all or groups of channels outputs to the mains! Holy cow that would be lovely.
