/CD requires receiving while transmitting, which is probably going to get really complicated in software when trying to precisely time GPIO pulses at .1us intervals on a 133Mhz CPU. Probably need to add an external latch + carrier sense circuit to assert a collision signal.
Side note, I do love the Firesign Theater reference, Mr. Tirebiter.
..because, now, YOU can whip up the Receive end, eh? ;-)