The ECU wants a 0v-5v pulsed signal for the speedo. It will get angry if you don't feed it a VSS signal. It will not be worth the trouble to try and get the DCCD working as it wants signals from the ABS computer. Better off running it unplugged, making your own locker switch or buying an aftermarket controller.
The only other thing I can think of that the ECU would miss would be in tank sensors (which you say you are retaining).