I don't do DCC, but think that if you start fooling with the voltage, unless you are running your trains at full speed, at first it won't have have any effect because the system is set to operate over a set voltage range, probably fairly narrow.
Then when the voltage is low enough to have effect, the electronics will crap out/go haywire.
But I can envision a way to make this work for you.
You would use a relay between the decoder and a motor lead. In the normally closed position, the relay lets current flow just as designed. In the open position, current would flow though a resistor/diode array before reaching the motor.
The relay would be tripped with a reed switch placed under the engine and a magnet in the track at the desired locations. The relay would be held closed for a moment by a capacitor that charges off the voltage and engages with relay closure. During this time, the train is heading into a block with no voltage applied so that it will stop until this voltage is applied, which would be tripped on by the incoming train, similarly equipped.