As per a previous post. The Ionic commercial brushes have the option for fan or pencil. when using the fan option. the spray of water is aimed just above the brush, across the whole width of the brush too. They work extremely well, with little risk of over spray.
They are great for most types of work where the window is directly above you. No good for over extensions, conservatories etc because they weigh a bit more than what we are now used to.
I asked Gardiners a while ago if they had plans to have a similar set up and the answer was negative.
If I were you Peter, I would look at developing a "clip on" device for the goose neck/neck with a john guest fitting that would allow the user to unclip from his pencil T join and clip onto the overbrush fan. Would only take a few seconds, no added weight worth mentioning and they can carry on with whatever brush, neck they prefer to use.