Mrs. Patrick de Rocher (the Frederica von Stade role) has two arias. The first one, the clemency hearing aria ("I am a mother who is asking you to spare her son's life") is rather long, but very moving and dramatic. It's also interrupted at one point by the victim's father, and I'm not sure yet how you'd cut around that. But I am going to figure it out, because this would be a great audition piece!
The other one is "No, son, no ... don't say a word" from the last family visit scene just before Joe's execution. It's easily excerptable, and is sort of a blusey Southern lullaby.
I'm sure Sister Helen has some excerpts that would work, too. The score is not available just yet, since there were some word changes after the NYCO production, but it should be available soon.