You know, you could use a folder if you set one up on a tripod unloaded, set the filter in to the camera and then set it in several positions that you think you might find useful by using a ground glass on the back of the camera then mark the viewfinder so you know where the gradation line is by using the lines you put on the viewfinder. True you will be shooting sort of blind, but you would just have to check your shot by holding the filter and looking through it before you mount it on the camera.

Just a thought, I hike and I know that every gram counts, but really, you are caring a Pentax 67II into the woods, that's a lot of camera to carry into the woods, that has got to hurt.