on my dell's docking station, there is a separate power button. maybe your laptop's dock or port replicator has one too. If you can find a schematic (or even a dock to take apart) maybe you can find out which pins on the big docking station plug get shorted for power. In many cases, it's just one pin that gets shorted to ground.
Heck, you might even be able to use a meter to test for continuity between the existing power button and pins on the dock port. just make sure all batteries/etc are removed from the laptop when probing the connector. the first thing i would test is to see if either of the power button's pins are already a ground by checking one side of the switch against the metal shields inside the laptop, then start probing the dock connector against the other side of the switch.