"Still, I can't envision Woody or Pete or even Harry Chapin on stage with an opera singer."   Why, because they don't bathe or something? What is wrong with opera singers?

Would Paul Robeson count? Not quite opera, but close. I believe he sang at benefits with both Seeger and Guthrie. Harry Chapin was a strong supporter of the arts in all forms, including opera. Because of his work, there are several companies in Long Island that continue to present their productions to this day.

I wonder if there is a opera forum somewhere that critiques Placido Domingo for singing with John Denver??? I would doubt it, I think lovers of opera tend to be more open-minded than folk music fans.

What a person does for their art should not stereotype them. Frankly, I think people tend to catagorize and compartmentalize artists when they do things that don't fit the mold. Look at the person as a whole, and what they have accomplished for others on the planet before passing judgement.