Why would a LD be not crosscompilable with respect to Darwin?
Darwin doesn't use LD, it uses DYLD. That's why you often get linker errors trying to port stuff. Why do you think nobody has ported KDE?
There is a compatibility library, which works if you sacrifice a chicken.
Is there something special about its format ? (e.g. copyrighted?)
No, it just happens to NOT be ELF.