I am a complete machine learning noob, so this could just be my lack of skill. Having looked at most of the popular ML frameworks, none of them seem to provide an easy, functional way to implement a custom 'neuron'. That is:

  function (someinput):
      output = do_stuff
      return output

Which then gets passed on to the next layer. Again, it's quite possible I've just missed something, or there is some inherent ML limitation that makes this impossible. But if such a thing is possible, it would be a rather awesome feature from my POV.