In 1979, Oh Dawn! Inc. released "The Amazing Spider-Man & the Incredible Hulk" in "The Gamma Gambit," a short comic printed entirely on toilet tissue.
Frankly, I think the Incredible Hulk is a terrible choice for this comic, as he's likely the only mainstream superhero who's not toilet-trained. The comic was penned by Jim Salicrup and Michael Higgins and illustrated by Marie Severin. Read the whole bizarre script here.