It's possible to seed a barcode, but it doesn't seem to be possible to seed something to say there is no barcode.

We could make it interpret barcode="" as "no barcode" but I think that would probably cause more problems than it solves, since something which used to be ignored would start being interpreted as no barcode. The other two options I can think of are a special value (e.g. barcode="[none]") or a separate key (e.g. no_barcode=true).