Construct RFID tag to turn on/off?
It might actually be just a little easier to make your own RFID in this case.  You will need a very low power microcontroller (an attiny or small PIC is typical) with an antenna and a small number of discrete components.  Here's a link to something similar: http://micah.navi.cx/2008/09/using-an-avr-as-an-rfid-tag/ 

With that, you can arrange for the RFID micro to power on when it gets power from the antenna, perform its sensory operation, and then respond based on the result of the computation.

Obviously, the available current from such a setup is pretty small, so unless the sensor works well on very small voltages and currents, you may have a hard time without an external power source.