I managed the speed on final one day without having the approach phase activated, the engines instantly powered up and we started accelerating. The Master Warning went off as flap limiting was exceeded before the head of the training department who was in the left hand seat pulled the speed knob. I was rewarded with a large number of extra sectors added to my program.
Also worth noting that if you are on an arrival and a speed constraint has activated below 10 000', say 220kts, if you go into heading mode the speed will increase to the default of 250/10 000. Worth keeping a look out for.