Here's something I caught from Mark Evanier. It's a few clips from a recent concert performance of the musical 1776 in New York. No costumes, and multi-racial casting. I'm not sure how to parse a black person playing Richard Henry Lee, still less Martha Jefferson, but it certainly works for the Courier who sings "Mamma Look Sharp". But the performers do seem to be all very good. I'm actually glad I didn't know about this before it happened, because I would have felt frustrated at not being able to go.
For true fanatics, here's another chunk of some of the same clips but also some different ones. And a third set. And some cast/staff interviews.