in principle I think it should be possible to do a shallow copy on a mesh to swap out spesific custom-data layers (vertex coordinates in this case).

It may be that there needs to be an API call to create & free shallow copies to prevent mistakes and avoid having low level custom-data handling inline as it is currently.

BKE_mesh_copy_for_eval is reasonably expensive, copying animation-data, ID-properties, materials, deform-groups, initializing thread mutexes, adding a session UUID ... as well as BKE_library_foreach_ID_link() that walks over materials, animation-data, id-properties etc, although not recursively.

Suggest either of the following:

• Support a shallow copy API for mesh geometry evaluation which provides a convenient way to make a short lived shallow copy to be used & freed immediately afterwards, allowing custom-data layers to be replaced.
• As this function happens only to be used from RNA, commit this patch with a warning this function should not be used during runtime/animation updates/evaluation and use is limited to the RNA-API, also an explanation for why BKE_mesh_copy_for_eval is used with a reference to the crash. In the future storing vertices as a float array means coords from either the key-block or the mesh could be used interchangeably.