Geometry swapping has become a lot less useful with the advent of weight mapping. The feature you show works for loose objects that do not bend.

In traditional rigging the 'weight' of a vertex (I.e how much it partakes in the bending of a joint) is determined by the group and the xyz position relative to the angle limits and the falloff zones. You can therefore replace one object with another because the weights of the vertices of the swapped geometry can be calculated from their xyz positions.

With weight mapping weights are defined per vertex. You would have to redefine the vertex weights upon the geometry swap.

And of course Unimesh will not work either.

If the alterrnative geometry is to be defined in the figure anyway, you can of course just define it with weight maps, morphs and all, and make alternatives transparent or visible (when body part has both a parent and a child in the hierarchy) or just hide/unhide if it has only either a parent or a single child. However it is more convenient to use conformation.