I was able to make the 3c509 driver work with a 1992 era Etherlink III card, but I can't get it to work with a 1994 era card (3c509b).
The card is identified and ping and telnet are working just fine.
For some reason, I'm unable to get NFS working either via root or while mounting after boot. The mount successfully occurs, but any attempt to use ls against the mounted directory results in server not responding. A brief review of a tcpdump of the packets appears that they look normal.
I'm not leaving out the possibility that my conversion of the code is not 100% right.
If you're interested in my patch so far, you can retrieve it from http://www.twosillies.com/webpal/3c509.c.patch
(With the NE2000 cards from halted.com being so cheap, it's almost not worth my time to do this, but I do like a challenge).