Maybe, but I think Koch was really curious about people and the human experience. I mean, why else write books on how to teach children how to write poems? I don't see a real contradiction between cynicism and positivity. I for one, have been characterized as an upbeat cynic on numerous occasions. Yes, we are burdened with the errors of our forefathers and ourselves and are lucky to have stumbled so far, but what luck and what incredible roller coasters we fall onto! All three deal with the particular weirdness of being a human being through a well-conveyed sense of humor and had remarkable insights on what it was to be human, not only in their time, but human, period. While they might not accept invitations to a dinner (especially one held by me) I think that if they were all in a room together, that there would at least be some interesting conversation. Vonnegut's The Sirens of Titan and Adam's Hitchhiker's Guide to the Galaxy both contain elements explored in Koch's own work, notably One Train May Hide Another where the life reveals itself in unexpected and remarkable ways. I'm sure there are many other connections to be drawn as well and unless I am very mistaken, I'd guess that each are fairly familiar with at least the gist of the other's work.
I love Kenneth Koch - I've said this before in these posts. You are right about his wonderful book, Wishes, Lies, and Dreams. Are you familiar with his book about teaching senior citizens in a nursing home to write poetry. I believe the book is called I Never Told Anybody. (1978) I'd definitely like to have Homer (if he existed) to dinner. Just picked up a new copy of The Iliad to use as I make my way through mk's Odyssey. If you go to this link, start at the beginning.