I just did DO BLACK PATENT LEATHER SHOES REALLY REFLECT UP? this summer and the audiences all loved it (so did I, but the rest of the actors didn't.) It continues to have a long and profitable life in regional theatres, and the original cast featured talented unknowns like Jason Graae and Vicki Lewis. It's a wonderful show about the mythical old style of Catholic-school upbringing, where nuns could still get away with slapping children with rulers and telling teenage girls things like, "Never go on a date to a restaurant with white tablecloths, because that reminds the boys of bed."
As far as unknown musicals--how about WINGS, THE MUSICAL, a triumphant adaptation of Arthur Kopit's play about a stroke victim? It's a tremendously challenging role for a middle-aged actress/singer, who must memorize such lines as "Malacats on the forturay are the kesterfats of the romancers"--and then sing them. (The show is much better than that just sounded.)
I ask in all honesty/What would life be?/Without a song and a dance, what are we?/So I say "Thank you for the music/For giving it to me."