I've made my own post here: https://itch.io/t/5444554/conditional-collections#post-14588504 about what you're talking about! I have a working implementation but it's not a scene injection mod. If anyone has the time or inclination to compile them into a working .mod.tscn file, that'd be super cool. I might be taking a crack at it if I have the time but I'm not super well versed in Godot and don't really have a reason to (since my version works).