Very interenting problem!
Do you know that there is a "Serial Over Ethernet" solution?
May not solve your problem, but can offer options...
The first problem is this to known if the BIOS is loading something or not.
Can you boot and ping the box with a standard TC?
In the "old days" it was possible to put the kernel + initrd in the boot partition, like:
cat kernel initrd > /dev/sdb1
The partition of course must have "the boot flag on".
Make an initrd that starts the network, and see if you can ping.
This does not solve the problem with TC, but can help you to find out if the BIOS bootloader is working or not.