Tucker brushes, (the original wfp) from back in the 1950-60s brought out a brush with outer bristles and a foam centre which acts in the same manner. Ionics and Tucker have the facility to rinse above the brush from jets fitted above the brush and the trouble with that is three things:
1. on a windy day water will get blown above the window frame hitting the soffits, air vents, etc causing dirt to run down onto the glass after the water has dried ending in customer dissatisfied with quality of clean, Even if it's not windy: it will be hard to avoid hitting soffits, air vents and lintels simply because fan jets or pencil jets situated above the stock or at the top of the wagtail will be harder to control as limited space on some window frames. Can see the advantages of using it on solar panels, large shop windows and bus stop shelters though!
2. Foam will trap dirt and possibly any loose cement, snails, etc and possibly scratch glass, no thank-you!
3. wouldn't shift spiders nest in top corners, clean frames properly and would trap dirt constantly so you would have to keep removing clogged brush or wagtail.
Love how easily the wagtail swivels though, nice concept but not for me, I'm out!