The Mac version comes with Universal Binaries for Csound. This means that you get both 32 and 64bit libs included. One could strip out the 32 bit binaries, that would make things smalled.
Exporting from Windows to OSX may work, if one includes the OSX and WIndows binaries in the StreamingAssets folder. I've though about doing this before, but it makes the package bigger again. But it should work. The CsoundUnity scripts check what platform is being used before it tries to load Csound.
As an ardent Linux user, I'm embarrassed to say that I've never even tried CsoundUnity on it. It should work fine, but we'd have to check if bundling Csound into the StreamingAssets folder works.