Well, I've been doing some playing around with networking in qemu on macOS, and found that when using -netdev user, smb= seems to map the host path just fine, but there are failures when adding smbserver=, and you can't address the virtual CIFS server without it.
But that doesn't seem to matter. The host is available at 10.0.2.2, and all TCP and UDP services appear to be available. So if you enable AFS or SMB on the host, you can connect to it using Connect to Server... in OS X Finder, and using smb://10.0.2.2
Under OS 9, of course, AppleTalk is too old a version to communicate with the modern Apple Filesharing Protocol. So far, I haven't got DAVE to talk to 10.0.2.2, and enabling the FTP server on the host and using FTP (via Network Browser or Classilla) also appears to fail. FTP failure is expected because the server attempts to connect to the client; SMB appears to fail due to some failure with UDP (even though it's supposed to work).
If anyone has any tips here, let me know