How about a package (or packages) for modules that support experimental GHC-only language features, like Generics (currently in base) and Template Haskell (currently in haskell-src)? This would make it easier for other implementations to ignore them (they do tend to move around a bit) and also present a distinction in the library documentation. (It might even speed up the GHC build a tad, because you don't need the rest of haskell-src for stage1.)