@rcfa:
But if I can't be 100% sure that a specific physical port is always going to be the LAN because at boot time the system may discover one or the other USB interface first…
Shouldn't, though I stay away from USB networking at all costs so I'm not sure, but it should be like PCI/PCI-e/PCI-X, where it'll never change from what it is (unless you rearrange the cards in that case), the detection isn't some random process.