Hmmm....
It occurs to me that another option might work. Given that the docking connectors are available (or could even use a smaller, totally different male/female type), why not make a separate docking connector for the unit and sled?

From what pics I have seen, it looks like there might be sufficient room directly above the ethernet connector or perhaps just to one side of the USB connector. Adding another docking connector there might be possible. Would it work to wire the existing ethernet connector in parallel with another connector? So both co-exist?

Then adding the corresponding dock connector on a sled would be trivial. And the new connector on the unit would only connect to a suitably modded sled. An unmodded sled would have no problem with it.

Would this work?