Actually, what we want is exactly what we currently do: native Windows dll already call into ELF libraries (albeit special ELF files).
Yes, but we can do this because we control the dynamic linker. The only reason I can think of (are there others?) to build the DLLs as real PE DLLs is so apps that do their own linking can work.
But then if they are doing their own linking, how do they get connected to ELF files?