I reached out to Oliver Schmidt, maintainer of Contiki for Apple 2 (among other things), and he suggested the following:
You can't "emulate" an Uthernet card with an RPi but you can -
generally - use the RPi to get online with Contiki.
* Warning *
I haven't tried what I describe below myself. And I have a report that
it doesn't work reliably. And it's quite some effort. Nevertheless...
Instead of using an Ethernet card Contiki can be built to use a
SuperSerialCard to connect to a SLIP server. There's no pre-built
Contiki for SLIP, you have to build it yourself.
"The cpu/6502/ directory is used for targeting 6502-based machines
using the cc65 compiler http://cc65.github.io/cc65/."
WITH_SLIP Default: 0 Purpose: Use SLIP (based on RS232 driver coming
with cc65) instead of Ethernet.
"The 'disk' make goal requires AppleCommander 1.3.5 or later. It is
available at http://applecommander.sourceforge.net/."
You can either build the individual Contiki programs using e.e.g
or build them all using
Getting the RPi ready is supposed to be easy, e.g.
says that something like
sudo modprobe slip
sudo slattach -L -s 38400 -p slip /dev/ttyAMA0
sudo ifconfig sl0 192.168.5.1 dstaddr 192.168.5.2
Please note that I don't know more about this then what I wrote above.
Good luck. Oliver
I will update this with my progress doing so, and try to post the rebuilt Contiki binaries somewhere as well. Thanks to Oliver for his assistance.
Update: I was having trouble with the compilation, so Oliver was kind enough to compile for me and send along. Here are the compiled binaries if you are interested.