OK, a little more progress, I have written a fake device driver for my 'emulated' network card, and that passes data into the empegs network handlers in the same way the existing ethernet port does. When I start emplode looking for the device over Ethernet this is what I see from my debug:
RCV: LENGTH 3a
Packet of length 58
ffff ffff ffff 0009 5b03 7bf0 0806 0001
0800 0604 0001 0009 5b03 7bf0 c0a8 0064
0000 0000 0000 c0a8 0069 80d8 2810 0001
0000 0000 0001 2046 4844
netif_rx()
no backlog, and not dropping
Dequeueing SKB packet
type is 1544
pt_prev = -1072769308
Send
Packet of length 44
2a00 0009 5b03 7bf0 0009 5b03 7bdc 0806
0001 0800 0604 0002 0009 5b03 7bdc c0a8
0069 0009 5b03 7bf0 c0a8 0064

It looks like the send packet is a response back to Emplodes request, but my Empeg never turns up in the discovery window. I suspect at the moment that there is something wrong with the USB response I am sending, so I am looking at that, but can anyone confirm that this is a correct response that Emplode is looking for?