I don't know exact sementics of Storable, but it's addable anyway.

instance Storable Ordering where
   sizeOf _          = sizeOf (undefined::HTYPE_INT)
   alignment _       = alignment (undefined::HTYPE_INT)
   peekElemOff p i   = liftM (`compare` (1::HTYPE_INT)) $ peekElemOff (castPtr p) i
   pokeElemOff p i x = pokeElemOff (castPtr p) i (fromIntegral (fromEnum x)::HTYPE_INT)