This idea of indirection is nice but your goal could probably be achieved more easily by introducing the strorage of selections. A plugin could even manage some Store sélection... and Restore selection... menu items without the need of changing the program. Saving the stored selections in a SH3D file might be more difficult though.

I could see this being implemented by walking the current selection items and adding them into a collection of weak-references. Restoring a selection would then just be a matter of setting the selection back to the weak-references that still refer to something.

I don't see a easy way to persist this into a separate non SH3D file.

A non-persistable weak-referenced based plugin would appear to be doable with very little coding. But I'm not sure it would be all that useful.