The engines will be at idle thrust at which time the lock will open and allow reverse thrust application.
Not so, I fear. In a rejected take off from high power, the N1 will have dropped to around 50-60 percent N1 on the way down to idle reverse N1 by the time the throttles hit the stops and the reverse levers are raised.
The action of lifting the reverse levers towards the first detent is what brings up the autospeed brake - not the idle N1 setting. That is why for landing, as soon as touch down on the main wheels occurs reverse should be selected rapidly to take advantage of the relatively high N1 occurring as the engines wind down.
If the throttles are closed at the flare and touch down occures (say) three seconds later the N1 at touchdown will be around 45 percent N1 on the way down to 31% N1. If, at four seconds after touch down, reverse has not been selected, the idle N1 will then fall to ground idle of 22 percent N1