I can confirm that it is possible to modify the key to the older PHEVs.
My PHEV is delivered in November 2013 and I have made a modification to start the heater. But it uses SW4 for activation.
2 clicks (as the remote tailgate) does this: lock your car and turn signals do 3 quick blinks, and the heater starts.
I bought a new switch and another shell with an extra button, think it is used as a panic button in some countries.
Then I soldered the switch between the minus (-) and TP6, which goes to SW4 ...