I've used a Bitzenburg to put on 7 1/2 fletching. It's a bit of a pain though. I clamp an Olfa knife blade in the front of the clamp along with the feather, just as a guide to keep the front of the feather straight. The edge of the blade can be positioned to sit up on the edge of the quill to put downward pressure on the part that extends past the clamp. Once you rotate the shaft, you have to shim it up under the front cradle with a pencil or arrow shaft so that the previously glued feathers will clear the front cradle that the shaft sits in. You have to be careful to keep the shaft centered at this point, because the cradle would normally do that, and now the shaft is sitting above it.
Like I said, it's a pain, but probably more accurate than I could achieve by hand, just eyeballing it.
I've never seen a feather chopper that long, but I would think that they exist somewhere...
A rotary knife works very well for hand-cutting them. Because there is no drag from the blade, you get a nice clean, straight top edge.
