A very thought provoking and interesting article Graham, and well written as always.
I have some disguised leads, and in any case I always use the brown ones, which may be Korda or other makes. However with 4ft hooklengths I doubt it makes that much difference. It's easy to forget the speed at which river fish like barbel and chub live their lives. They are not in the least like carp and tench, patiently chewing the cud on the lake bottom. It's much more necessary to have short hooklengths - and therefore visible rigs in the baited area near the hook - so disguising leads may become more an issue.
Nevertheless there is no reason for NOT disguising leads for river use, except laziness or lack of time, so if I get the opportunity I will paint or dip them all... However there is not much that can be done with feeders is there?