And a minimal respin of that board ought to be much cheaper in R&D than creating the original from scratch once was..

Unfortunately, a minimal respin isn't going to get you 100Mbit Ethernet. Even if someone could dig up a non-PCI fast Ethernet controller, the car-player does not have enough bandwidth in through Ethernet through the CPU and out to disk over PIO to saturate 10Mbit, let alone 100Mbit, and you would see no increase in transfer speeds.

Peter