Shading World UI

How would one try to render a UI but post processed, i want the UI to be shown CRT style on a ingame object.
How would i go about implementing this?

The easiest way would be that the UI is just a texture i could use in a material.

But i'm unsure on how to render UI to a texture. From what i can tell most UI code for rendering is internal and thus unable to make my own world UI renderer that does shading too.

Sounds like you are trying to do Post Processing effects on a WorldPanel since it's on an "ingame object"

This has been done before, you should check out this library:
https://github.com/jammees/Postprocessable-Razor-Panel

Alternatively you could look at https://github.com/tzainten/VividPanels