You need software to drive that ethernet port, some of it is probably included in your player. If these support the standard TCP/IP protocol, there is hope (they probably do).

There do exist OS9 clients for telnet, ftp and even nfs. I never tried the latter one, though. These were included in packages named something like OS9/ESP, OS9/ISP or UniBridge.

 IIRC it was explained in another thread on a forum that the card in your 604 wasn't an ethernet card, but T1. And didn't the card write video images directly to memory/display from the network? To quote: "The two boards where special made by Imagenet.
The top board with T1C1 has a special connector. It's for a T1 telephone line. The C1 stands for the image stream from the server.". So it sounds to me like it's directly streaming into video ram and is not accessible from the OS.