RFID Reader..
Yes it should be possible, Most Tags use ASK or FSK modulation (With Subdivision of the Carrier frequency) to receive instructions from the tag. The tag then responds with ASK on the CW carrier produced by the reader.
All you should need is the command set for the tag you are talking to! Some tags just need a CW carrier and they will give back the data they contain.
The problem come from the design of the reader, Most are made from ASIC's with a simple serial data in/out interface for transmiting/receiving from the tag. These ASIC's will talk eith ASK or FSK to the tag generally on one frequency so if your Reader uses an ASK ASIC you wouldn't be able to read an FSK tag without hardware mods... :sm35: