It's an interesting idea that I hadn't thought of. There would have to be an atomic file system operation to "commit" a compiled module - getting that right could be a bit tricky (compilation isn't deterministic, so the commit has to be atomic).
I suppose you could just rename it into place when you're done. -Edward
I was imagining that it could create Module.o.compiling and then rename into place when it's done. Then each ghc would do a work stealing thing where it tries to find output to produce that doesn't have an accompanying .compiling, or sleeps for a bit if all work at this stage is already taken, which is likely to happen since sometimes the graph would go through a bottleneck. Then it's easy to clean up if work gets interrupted, just rm **/*.compiling