Well that's a surprise
I just picked up a copy of the newest Dark Tower comic book (4 of 7), and I found that in the back there's a transcript of the Q&A from the Dark Tower panel at New York Comic Con. Here's the last bit.

Audience member: With the world of the Dark Tower being one where other stories like to find their way in, like Wizard of Oz and Harry Potter, can this new series see any of the Marvel Universe creeping it's way in?

King: What was your question? I'm sorry.

Audience member: Are there going to be any Marvel characters in the comics?

King: It's really sort of a group effort at this point. If they came up to me with that idea or if I came up with it, we might toss it in the mix, but don't mistake us for people who know where we're going.
[Audience Laugher]

That was me. My embarrassing interaction with Stephen King is my first appearance in a Marvel comic, credited as "Audience member." RADICAL.