The ACL's are handled with a separate protocol which is IMHO the best way to handle them. One only needs to look at the SMB^H^H^HCIFS protocol to see the wisdom of doing it this way.
Well, maybe. One might also look at said protocol to see, instead, the wisdom of avoiding its mistakes; I'm not sure I'd say putting ACL support into that protocol is one of them.
The protocol is even rudimentally described in /usr/include/rpcsvc/nfs_acl.x.
Well, that's the way Solaris 2.5-and-later handle them. Digital UNIX has a different protocol, which I think is actually an "extended attributes" protocol.