I just did a test. In all cases the car was locked (P,R,N,D). In that case the trunk would not open with the press of the outside button. If the car was unlocked and in neutral it would open.
So it seems to boil down to the car being locked or not. Since the car self locks almost immediately I think that the integration with the canbus/diagnostic is overkill. I simple parallel switch on the same circuit as the physical switch seems adequate to me.