I have nearly the same plan: I want to compile a restrictive form of Haskell to constant time and space C code for hard realtime embedded targets. Except I need a top level monad with different semantics than IO. Which language is that? ImProve?
Wouldn't a fake driver (fakeRun :: Something -> IO ()) at least simplify the problem? It would prevent you from having to modify GHC to handle a different top level type. And during your evaluation of Core, you would simply ignore fakeRun. I'm not really bothered about the type of main, since I can always just pick a different unused name for my entry point. In fact, that would allow me to compile programs both as normal executables for functional testing (using your fake driver solution) and still pass the Core to the rest of the pipeline.
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