I'm the author of Quest - thanks for linking to it. Feel free to ask questions about it. If anyone wants to try it: it's implemented in OCaml and can be installed from OCaml's package manager Opam: "opam install quest".
Really cool, I did a quick search before I wrote my version and didn't see it. Thanks for writing quest, I will add it to my toolbox.
Quick anecdote: I always demoed Quest by running it in a loop testing GCC on an Apple laptop and it would always find a compiler bug. No longer, it seems. I've tried it on OS X 10.10.5 today and so far couldn't find a bug in GCC or Clang.
I think the interop between compilers may have more bugs since that isn't tested for complicated struct args much. Another good place to try is the tcc arm port.