You can achieve this with Animation Nodes or Sverchok.
I actually made a morphing modifier long time ago, but it was never accepted into Blender’s main. The morph modifier basically was just a modifier that could be put in any order in the stack and accepted weight maps. That way you could run the Vertex Weight Promity first then use this modifier underneath. Unfortunately it is not in the current builds.
It is possible to use it to patch Blender and you will have the morph in the actual stack.