I believe the way it works (though I could be wrong) is that an open-source organization that signs up is supposed to have projects that concerns its codebase or related components. That means either the toolchain or some related tool like a debugger or perhaps something like Merlin. Perhaps it could even extend to a library that can be shown to be extremely useful to the community -- something like Lwt or Batteries.
It would be harder to make the case for something that's purely an application on top of OCaml, though I do believe that is possible once an organization is more established with GSoC, for example Xen getting Mirage into GSoC -- Anil would probably know more about that kind of thing.
Again, I could be wrong about this, so if anyone has better information, please correct me.