Most likely anything you do in this arena will be custom-made. I don't know of anything pre-existing for RFID, although I've done some pretty neat stuff with card and barcode reading before.
You'll probably go one of two ways:
1) writing your own code that can be executed via AppleScript or Send Event to perform the write.
2) Writing your own plugin (IE via Java/groovy in scriptmaster) to perform the write based on an external function executed from filemaker.
There are dozens of writers out there, so you might want to invesigate which software seems the most command-line friendly for writing, and use method #1 above.
A lot of others might find this cool, make sure to post some details about your solution if you custom make something. You might drum up some side business for yourself.
I looked at Phidgets... they don't have an RFID writer, only a reader.
Called them up and they said maybe in 2 months they'll have one.
In the meantime I found these guys;
Now my problem is writing some C or Java or something code that's going to make this work.