Is there a MacOS distribution of Eiffel that anyone knows of ?
Nothing recent. SmartEiffel did have old MacOS ("Macintosh") support, and recent MacOS are just BSD with another name and specific conventions. It should not be too hard to have a working compiler.
Libraries are another problem, the same one as Windows actually: the core libaries should work but anything developed or changed in the last few works is expected to work on Linux only because that's the OS we hack on.