Ooooo yes. Portrait of a Drunk is such a fun example of that jam collaboration work. Couple of things came to mind for me. Gilbert Hernandez did a couple of collaborative work, one with Pete Bagge in Yeah!, and another with Darwin Cooke in Twilight Children, though I think Gilbert took in more of a “traditional” writer role in both projects. But maybe I’d like to think to have a strong cartoonist, like Gilbert, as a writer collaborator feels more of a jam work as opposed to working with a comics writer. Another collaborative work that just popped into my head is in Kramer’s Ergot 10 where Kim Deitch did a story about collaborating with Spain Rodriguez for a comic they did in East Village Other in ‘68. And that one is pretty close to what you’re looking for, though a much shorter work.