I realize the effect could be called a "soft merge" or "reversible merge", and might be useful for other things. You could use a similar effect for sculpting multiple objects as a combine mesh, complete with everything you can do in sculpting (except re-topo, of course) and take the sculpt result back out to the original mesh(es). In this case, the lookup data needs to also include the mesh id. Each "unimesh" vert would have a list of original mesh_id.vert_id.

I must be up the wrong tree. This seems like something obvious enough someone would make it :o