Eris
I have the original published script and it says that the jacket art was copyrighted in 1980 by Frank "Fraver" Verlizzo. I assume that's the guy. Hope I'm right :) I don't really know anything else about artwork on Broadway at that time. But there are books that can inform you. Check your library and look around in the Broadway/Theater section. Hopefully you'll find some books on it. Also, in this book of the script I have, pages 10-16 have printed drawings of several characters/actors from the Broadway production. They are Al Herschfeld drawings. I do know that he is famous for his drawings of Broadway stars and celebrities. Look up some of his stuff online. They're a little different than the cover art for the ST playbill, but they are still really cool. Sweeney Todd is my favorite musical of all time!