Find the original blueprint when "Select new contents for the blueprint" is clicked

[DaveMcW]

When a player clicks "Select new contents for the blueprint" to replace an existing blueprint, the on_player_setup_blueprint event fires, and all the entity data is stored in the original blueprint. Reading the original blueprint is the offical way to access this data. However, the event has no way to access the original blueprint directly.

What is the best way to find the original blueprint?

I tried saving the original blueprint in on_gui_opened, and this works when right-clicking a blueprint in the player's inventory.

However, it fails when you open the gui by:
- Right-clicking a blueprint book in the player's inventory.
- Right-clicking a blueprint in the player's blueprint library.

Is there any way, or combination of ways, to reliably find the original blueprint when the player clicks "Select new contents for the blueprint"?