It is possible to use any $FF...$ variable as part of the barcode test.
However, you may find that this is also problematic since we do not format that string as it may be needed by the barcode component.
If you want to automate this you may need to pass the information to a script via the external component and then feed it back to FFCore (using manifest submission) after manipulatings the strings for the dimensions so they match finisher requirements.
Can you post requirements for the finisher?
