Thank you for the feedback and sharing that tutorial.
I used a different approach, but I am surprised that I did not come across this tutorial while trying to implement the effect. My solution used this mask shader to implement the technique - funnily enough, the topic for that thread was discussing how difficult it is to mask sprites in Godot. I came close to giving up before finding that.
If you are interested, you can see my shader here and the script which supplies the shader arguments here :)