I did a little bit of work on this a while back and will be returning to it soon but there are a few gotchas:
Amongst other things, I think that the Ethernet interface can only use fixed IP addressing and not DHCP when PPP is being used.
I think I also had to change net.lib (in 10.66) as follows:
// Test if interface uses ethernet NIC (we don't count PPPoE, even though it uses ethernet)
// PMCS #define IF_USES_ETH(i) ((word)(i) < USING_WIFI)
#define IF_USES_ETH(i) ((word)(i) < USING_ETHERNET)
as the way the test was done was not correct but I can't remember exactly what was wrong.