[SOLVED] Is it possible to use the GHC API to modify a program while compiling it?

I want to test the implementation a compiler optimization by piggybacking into the GHC compilation process and altering its Core representation. The idea would be to have something like:

runGhc (Just libdir) $ do
   ...
   c <- compileToCoreModule targetFile
   compileCoreToObj False (modify c)
   ...

where modify takes the Core representation and returns the modified version. This code, however, fails (even when modify does nothing at all) with the not very helpful message:

expectJust mkStubPaths

Any ideas on how to make this work, or if it's even possible to implement a code transformation this way?