More work on Ethernet support last night. The W5100 raw socket code is in place, and the Cabletron EA412 driver attempts to load on the Macintosh SE. Unfortunately the Cabletron driver crashes (hard) and prevents MacOS from booting further.
The command sequence thus far is:
Get MAC Address
Sometimes I don't even see the Set Protocol command.
My next steps will be to try it under NetBSD, and to install macsbug on the SE so I can try to see where the Cabletron driver is crashing.