You can also use a modified MS13 for that. The MS13 uses 2 X10 addresses, one for motion detection (PIR) and another one for dust/dark detection.
You can then replace the varistance by something else (a reed relay, a pulse counter, etc) and everytime the resistance of the new circuit falls below some value the MS13 will transmit an ON on the second address (A1+1). On the opposite way each time the resistance gets above another value you'll have an OFF.
This way, you'll have nearly a free new sensor!