what about replacing the acl code with POSIX ACLs [1] and file system capabilities patches [2] ?
the append only flag is default under linux and the hide flag is in my opinion a security by obscurity trick..
filesystem based acls are also rapidly than configuration-file (/etc) ones.
another probably good thing could be the addition of some priviledged UIDs:
a system administrator uid, for daily use, without CAP_SYS_MODULE, CAP_BOOT, CAP_SETPCAP, CAP_LINUX_IMMUTABLE, CAP_SYS_RAWIO, CAP_SYS_PTRACE and so on ..
a network service uid for running network daemons with CAP_NET_RAW, CAP_NET_BROADCAST, CAP_NET_BIND_SERVICE
after an initial configuration with uid 0 we can use the new system administrator uid and work for lowering the priviledges to make our system immune to break-ins possible damages.
excuse me for the sometimes-unclear english