Well, it's another day and the desired result inches closer.
Here is a simulation of gravity with little cubic raindrops.
Particle and Collision Simulations in Geometry Nodes Blender 3.5 | Tutorial
The nodes build-up includes finding the reflection angle of a normal and other useful bits. The physics engine includes wind as well as gravity so It should translate from falling drops to horizontal streamers.
I should try this. Unfortunately, we're in paint drying season so I'm trying to take advantage of that.