We are together here. I see where you are going, but it may require more than meets the eye. How would you tell the devices apart before compile? You must select the correct Arduino board in the IDE before compile. It does not even do that automatically.
I thought maybe using a different ethernet library include file for each. I wouldn't want to include all the code for every ethernet device, just the one I am using.
#include <Ethernet.h>
// or
#include <Ethernet5200.h>
// or
#include <Ethernet28J60.h>
All could include the same functions with the same parameters and return values as the current ethernet library.
Ethernet.begin()
server.available()
client.read()
client.println()
client.stop()
...and the like, don't you think?