If I was going to do Conversation-only PBs I would really wonder what my main marketable trait of the book is.
It can work but the conversation would be secondary to the content of the convo, the way the conversation works to the story, and how the images and implied story add to the convo.
I don't know but I wouldn't be surprised if Conversation-only PBs wouldn't look too interesting on paper but coupled with pictures, would be really great.
For example
-I WANT MY HAT BACK is basically "Have you seen my hat", variations of "no", and "Okay, thank you anyways" and then at the end "I love my hat"... not much there but it's the implied action between the conversation that makes the story; particularly the ending.
-GREEN EGGS AND HAM is all convo but the selling point is that it's only 50 different words. A kid can learn to read and the repetition and rhythm are great. It's also a funny idea since the suggestions get more and more outlandish.
-DON'T LET THE PIGEON DRIVE THE BUS has the reader constantly saying "no" to the bird. It's sorta interactive in that way.
I haven't attempted to write a conversation-only PB, but I would definitely look at what makes these books 'tick' from a manuscript-only standpoint and then see how you can use that to add to your own.