I've been digging around looking for a way to add rabbithole interactions to shells within a world's map, but all I'm finding are tutorials for making plain rabbitholes and replacing the map image. Does anyone know how I'd go about editing the world's set-dressing, so to speak?

use TOOL to place a custom object to click on in the world, maybe ?

You would need to somehow identify the object you were wanting to turn into a rabbit hole and then create an override package using s4studio. You would then need to change the tuning, or add tuning if there is none, to point to the rabbit hole you want it to behave as. Hopefully it would be a simple as that but in some cases routiing and footprint data may be needed so that Sims know where to stand to be able to interact successfully otherwise you might get a route failure.