you could ask at some Mac forums. I guess there must be some filesystem for Mac with compression, but that will make it very CPU expensive. I don't relly like HDD compression (I've experiencied it on Win many times). It might become especially CPU expensive in combination with aMule since it will have to continuosly recompress large files, since aMule can resume a download from any part of the file, not specifically from the begining or the end.
Greetings!