Thanks all. Fixed. (Less onerous than I thought it would be.) Yeah, the barrel doesn't have nubs to prevent movement, but the logic seems to be that if the adjuster shank is unscrewed sufficiently, the plastic barrel has to be pulled up to engage it -- otherwise the barrel just turns freely. (This isn't particularly rigorous logic, if that's the case. Most people will probably have the shank screwed closer to the brake arm.)