How to protect/enhance an LDO so that it can sink current in the event of an external wiring fault?
I would just use the load resistor in the single 3.6V LDO supply rather than adding additional active components. 

The load resistor works by keeping your LDO in the active regulating mode. If the normal load ends up sourcing current back toward the LDO it simply stops outputting the same amount of current as coming from the load sourced current.

Adding the extra active component to provide the source/sink capability may be a decent cost trade off if you really want to minimize the power dissipation of the circuit. The load resistor I recommended above will have a constant power consumption and will add to the dissipation in the LDO.