Ah, good point. Yes - MeshMap is really just for points which stay on the mesh.
For transforming geometry that is also above/below the mesh, you could use MeshCageMorph
meshcagemorph_example.gh (20.9 KB)
(uses the component from here Skeleton fattener + mesh cage morph, and weaverbird for the thickening)
(you need to click the button input to the MeshCageMorph component once at the start to calculate the initial mapping)