Would a "twenty question" format be successful with interviewing experts? I think it's a great idea that will result in a successful interview.
A common format in magazines is to ask the same 20 questions of each celebrity or expert. This format appears very popular and well received, and I think it will save you a lot of time and effort.
Let's say you are going to interview 20 copywriters, and you are a copywriter too, that will make it easy. So you have 20 different copywriters and you have done another interview with someone else on copywriting and you have all those questions from the interview already. You can just put them on a piece of paper and it would be my basic questions so you can get any other copywriter and ask them the same exact questions. No need to make new questions for every different expert, you have them already all done and ready.
I think that would be a nice product because hopefully the questions you've asked are good, quality questions and you have really worked hard to do your research or even done an Ask campaign to get the questions that people who want to break into the copywriting business want to know. Why bother writing new questions when you have already put the time and effort into great questions on the topic. Now people get to hear the same great questions asked, and see multiple perspectives on it.
So asking those same questions to 20 other copywriters I think that would be great and very easy and less work for the interviewer.