Interesting thread. I think the only ones that really meet newintown's criteria are the Cyranos (1973, 1993), two Comedy of Errors adaptations (Boys from Syracuse, Oh, Brother!) and the musical adaptations of Three Men On A Horse (I love the score to Let It Ride!, btw).
I also thought of Lysistrata by Aristophanes. In 1961 there was a musical adaptation starring Cyril Ritchard titled The Happiest Girl in the World. The music was lifted from Offenbach themes. It ran a merry 96 performances. In 1972 there was a musical play of Lysistrata starring Melina Mercouri (she of Illya, Darling) and featuring original songs by Peter Link (King of Hearts, Earl of Ruston). It managed to last 8 performances.
Coincidentally, I'm listening to Earl of Ruston right now. Why hasn't Capitol or DRG reissued this on cd? I like the songs and their 70s arrangements. For that matter when is Clownaround going to claw out of the past and get issued even if it is digitally a la The Capeman? Me thinks a new thread is forming.
Thinking about this I was surprised there hasn't been another musical adaptation in some form of Twelfth Night. There is the lush Richard Adler composed Music Is, but I believe that is it. Correct me if I am wrong?
With Irma you gotta do something!