Darwin/XNU is not supported at the moment.
panicall has ported (video) syzkaller to
Darwin/XNU and that has found more than 50 bugs including
CVE-2018-4435 mentioned in Apple security updates.
Darwin/XNU is open-source and has KASAN, but no KCOV at the moment (though not required for intial support).
PureDarwin may be used to create VM images suitable for fuzzing.