I found something that sounds like this; it's called "Superfloss" (OralB product); they are about 10" long, with a 6" section in the middle that looks like a miniature pipe cleaner. Speaking of pipe cleaners, I also tried using them. They were a bit too big to get through the space, but there's something called "Chenille Stems" (Fibre Craft product), available in craft stores, which is like a light gauge pipe cleaner, but with longer 'pile'. It won't go through as it, but if you trim the pile, it will go through most slots (there is component variation, and for some blocks, the gap between the block and the PCB is much smaller). I'm not sure that would get off the impacted dirt. (At least, more so than a long soak, and then water jet.) I did try and find a small pressure washer type thing, but the smallest I could find was ... still pretty big. Maybe a Waterpik would be more the backplane's size.. Noel