Paper and pencil is the fail-safe solution (which I'd recommend). As a potential alternative or in addition, you *could* display the code in a <surveypage> / <textbox> element. Participants could then select and copy the code via the context menu:
Example code for the above:
/ blocks = [1=myblock]
/ trials = [1=mytrial; 2=codepage]
/ stimulusframes = [1=mytext]
/ validresponse = (57)
/ items = ("Press the spacebar.")
/ showpagenumbers = false
/ showquestionnumbers = false
/ questions = [1=codebox]
/ caption = "Select and copy the following code:"
/ defaultresponse = "ABCD1234"