I'll admit this is a somewhat strange choice, but I think Jerry Garcia may deserve a nod as an 'unsung hero'.
While clearly he was a very famous musician, his contributions to what I'd call 'American music awareness' are worth noting. Others have mentioned NRPS - clearly they wouldn't have existed without Jerry. He did an important job bringing American folk music to a new generation, and his work with Old and In Way, and Dave Grisman are great for finding lesser known treasures.
Don't get me wrong, I know all the bad sides (e.g. endless noodling). But I do believe his contributions in this area don't get the credit they deserve.
Curtis, This year at the AMA's there was a session topic "Is Blues Americana." I didn't got cause of conflicts in my schedule, but that's easy..of course it's Ameriana..at its finest. When I interviewed Michael Martin Murphy, he felt the two major strains of Americana music were country and blues. He felt everything flows from those two...Arguable, but the main point is, Blue is Americana. All of the guys on your list are great.
this thread has become so interesting, I'm going to try to talley up the most mentioned to a top ten and post some comments as a feature. This is just for fun...No big meaning to it and very subjective...but, if anyone lurks and feels strongly about chiming in.....Let's see who else comes up or who gets more votes.....so far The Blasters, Roger Miller, Mickey Newbury and Mike Nesmith are all looking good....
Unsung heros?... Influential? Hmmmm...
Lowell George My New Orleans discovery started w him
Jon Pousette-Dart strong regional acoustic act in NE and NY when I was a kid
Big Momma Thornton I just love her, and so did Elvis etc.
Paul Geremia Another NE guy who was big for me
Duke Robillard Yet another NE guy who was big for me as a kid and pays the rent now...