Can't you put the bark through a garden muncher? The sort of thing
you put branches through to turn into finer bits to use as mulch or
compost? I use a little electric one that doesn't have a mesh on the
bottom so size is pot luck, but the big petrol ones have a mesh. just
a case of finding a friend with one you could borrow. I've used
these things for branches and bark for work (extracting natural
chemicals) as well as at home and they do the job.
Andy