Darwin/XNU is not supported at the moment.
panicall has ported
([1]
(video),
[2])
syzkaller to Darwin/XNU and that has found more than
50 bugs including
CVE-2018-4447 and CVE-2018-4435 mentioned in
Apple security updates.
Darwin/XNU is open-source and has
KASAN.
Latest versions also contain KCOV-like support for coverage;
it's not uploaded to github mirror yet, but this
tarball
contains san/ksancov.{h,c}.
PureDarwin may be used to create VM images suitable for fuzzing.