We recently presented at the last Siggraph Asia a novel technique called “QuadMixer”, it is similar to MeshMixer but works wit Quad Meshes.
QuadMixer is a novel interactive technique to blend between base quad meshes preserving the majority of the original layouts. Our blending process is reliable and extremely efficient, which makes it perfectly integrable into an in real-time modelling system. Quadmixer allows to smoothly compose different quadrilateral meshes keeping as much as possible all the layout in the areas which are not involved in the operation. In other words, we enable for reusing of the previous work performed by the artist in the retopology/modelling phase.
You can have a look Quadmixer here:
I think it would be a powerful tool if integrated in Rhino. What do you think about? Do you have any contact with developers to start some collaboration?