I feel for you, lurk.
I can't seem to reach right between my shoulder blades and upper back very well.
Using the corner of the doorway is all well and good, but it's kinda inconvenient when I'm trying to get some work done.
I came up with a solution a few years back.
I forget now where I got it, but I found myself to be the owner of a short sword of about 18 or so inches long.
I had a bad itch and was very frustrated because it wouldn't go away.
I hit on the idea of using the sword as a back-scratcher.
It worked well for a few times, but then in a particularly enthusiastic moment of scratching I cut myself. Oopsy-daisy!
Growing more frustrated,(and determined not to give my back as thorough washing as it needed
) I had another rather brilliant breakthrough in back-scratching technology when I realized the little plastic report paper holder I was using, fit over the sword-blade like a sheath, but left just enough sticking out to give a good scratch without cutting me.
I used it and abused it 'til it broke.
Now I have to resort to doorjams and thorough backscrubbings.