Came up with an ugly hack that works.
I have a blueprint that detects screens and spawns my RenderTarget Blueprint per screen with a reference to each screen.
On Construction Script of my Render Target blueprint, I get the attenuation radius of the RectLight and store it as a variable.
Then On Tick, I set the RectLight’s attenuation on a FlipFlop between the default stored variable value and the default stored variable value + 1.
This is a very ugly solution, but for now, it works.
EDIT: You MUST set the Source Texture to the render target in a blueprint. If you set it as a default, this doesn’t work. Idk why.