I thought that too, which is a shame as you have to rely on the customer being home to be able to clean them! (Usually in the UK you just turn up, get out and use your van mount system, clean the windows and go to the next one, majority of the time not even seeing the customer.)
What Luke said is the easiest but if you had an extensible doodlebug or one of the water fed pole accessories that holds a white pad you could use it to agitate between the rails and then rinse a lot from above. I don’t believe it is worth your time and money but it could work.