Since they fixed the Area Node to detect concave collision shapes in 3.2.1I think I found a "hack". Area Nodes can't use concave collision shapes as their own shape but they can detect them using a primitive shape.
Instead of having an Area Node with the custom collision shape sit and wait for a PhysicsBody to trigger a response we can have a PhysicsBody with the custom collision shape sit and wait for a small 'mobile' Area Node (using a primitive collision shape) to trigger a response!
With a StaticBody holding the custom collision shape and the Kinematic/Rigid Body carrying a small Area node we can simulate the interaction akin to a player walking into a trigger area but instead the trigger area can be any shape we want to be!
Imma noob so I'm not so sure if this is something old or if somebody out there already came up with the idea so apologies if I sound too exited of coming up with this alone.