Associated data families and overlapping instances

I'd like a 'generic' map data structure that can be efficiently specialized by providing custom instances, much like in the GHC manual section on type families.

{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE UndecidableInstances #-}

module MapKey where

import           Data.Map.Strict    (Map)
import qualified Data.Map.Strict    as Map

class MapKey k where
  data MMap k :: * -> *

instance {-# OVERLAPPING #-} MapKey () where
  newtype MMap () v = UnitMap (Maybe v)

instance {-# OVERLAPPABLE #-} Ord k => MapKey k where
  newtype MMap k v = OrdMap (Map k v)

Sadly, this doesn't work. GHC (8.2.1) complains:

    Conflicting family instance declarations:
      MMap () = UnitMap (Maybe v)
      MMap = OrdMap (Map k v)
   |
14 |   newtype MMap () v = UnitMap (Maybe v)
   |

Is there some language extension which allows this? Otherwise is there another way to make it easy for users to define a 'default' instance for Ord?