It would still be incompatible even under Classic, because Classic routes through the kernel's networking for shared drives, not through its own. The last version of OS X that fully supported EtherTalk-based AFP (AFP 1.x/2.x) was 10.2. On that version, you could use the Chooser in Classic to mount a classic AppleTalk server, and OS X would use it. That doesn't work with 10.4 at all, and 10.3 only supports AFP 2.2, which doesn't work with many servers.