Hi there!
It already has this feature. You can use the plugin parameter to set a different skin on the Window_NameBox or use a plugin command:
Viewing post in Hakuen Studio Window Skin Manager for RPG Maker MV MZ comments
I think I figured out what happened. It looks like the plugin instructions don't include the Window_NameBox in the list of default windows you provided. It has Window_NameEdit and Window_NameInput, but not Window_NameBox. So I incorrectly assumed that it was not a feature since nameboxes are attached to normal windows