In RFID, are there tag modules that give the user a choice of different outputs?
RFID tags are simple. A dedicated microprocessor with an eeprom, and a coil. Passive tags derive their power from the RFID field of the reader, and then respond with their eeprom contents, either a serial number or string. Simple. You can make one with any microcontroller if you follow the standard. If you want a tag that can be selective of the string it outputs, then you use standard microcontroller circuits and programming to get that done. The RFID part can be just the microcontroller, a coil (or inductor), and maybe a capacitor.

A DIP switch and a few input pins:



Pseudo code:

   SetupRFID()
   While(1){
    ReadInputs()
      If Input1 + Input2 + Input3 + Input4 == 4
      then TagOutput == String4
      else if Input1 + Input2 + Input3 + Input4 == 3
      then TagOutput == String3
      else default TagOutput == String0
      delay for 1 second
    }


Aside from the link that @0x6d64 provided (http://dimme.net/rfid-tag-emulation-with-an-avr/), there is also http://scanlime.org/2008/09/using-an-avr-as-an-rfid-tag/ and http://hackaday.com/2011/09/26/barebones-pic-rfid-tag/