I've never done a combed backing. I've backed a bow with strands of hemp twine, by sizing the back and laying them down vertically and covering with glue.
My thoughts (again un-tested) are that the same technique could be used for raw hemp, but you'd have to 'comb' the fibers while there are wet with glue to get even thickness, and distribution. You may also have some trouble getting them to lay flat. I think literally a coarse toothed comb would work well for this. I'm sure a search would turn up some more info.
Gabe