When taking the Peshastin back way to Leavenworth I have always noticed the Derby Canyon road going out of Peshastin. According to my maps it takes a loop out to FR7415 and around through Nahahum Canyon and on in to Cashmere. It is color coded as a major connector, my question is is it paved all the way?
I had to do a double-take when I read the notion of Peshastin - Cashmere on an FJR forum because the context was all wonky for me. While my goal wasn't exactly the same as yours I think I did the exact route you're talking and barely survived
on a Husqvarna with knobbies in August. I did get some great pictures though!
Memory serves that just out of Peshastin it transitions quickly from pavement to gravel, by a weird farm with foreboding No Trespassing signs and lots of blue tarps, and then quickly to a forest service goat trail...and something similar on the Cashmere side. This accumulation and some of the pictures on the thread were from the NF-74xx and NF-900 area. It wouldn't be a place I'd take an FJR unless there were a ton of bonus points involved.