Instead of changing the “consolidate corner points” default of the box sop, maybe its better to add a parameter to the box sop, defaulted on, mimicking the “vertex sop” with “cusp normal” activated.
“consolidate corner points” actually breaks the points apart, while vertex “cusp normal” doesn't effect the geometry structure, while still allowing the expected normal display for the viewport.
A third choice is to also have a cusp normal viewport display option. The problem I worry about adding vertex normals is that box modelling after the fact can lead to unnatural looking normals. The problem with putting this as a viewport display option though is that it's probably slow unless there is much better support at only recomputing normals on only the affected/modified geometry.