Lukas,
My suggestion would be to approach the Open UI control to be similar to that of the barcode scan field. I would override the StringEditAdapter as you have done but for my field display both a text input area as well as a button. The button would call the RFID reader read function and then populate the returned value into the text field which would pass back through to Agentry.
I would have also expected to see you call the RfidTagReaderAPI.InitRFIDReader(); methond as part of the initialize function to setup the connection to the RFID reader. Then when the button on click is called you are ready to call RfidTagReaderAPI.Read15693TagID() to get the value from the RFID reader.
One thing you will need to deal with is what do you want to happen if the RFID reader gets a read on more than one tag at a time?
For the RFID readers that Agentry supports, if more than one tag is found they are simply concatenated together and returned as a long string value that we then parse in Agentry and act accordingly.
Hopefully this will help.
Good Luck!
--Bill