You can sort of do this, but as far as I know there are still some missing pieces to getting everything you would want.  I'm in the process of doing this for some STM implementations with some colleagues and what I have so far has a few changes to GHC (teaching the GC about some new heap objects) some cmm code in the library imported as foreign prim's and some C code in the library.  I build this with a cabal file and use the "--extra-include-dirs" flag to pass the path to the "rts/" directory for ghc.  This last bit is the unfortunate part that I imagine once we work out what all we need exposed, we could include in what is installed with GHC.  It might already be there, I'm not sure.

Ryan