Use screenshot as material during runtime?

You might want to look into Scene Capture 2D.
-Create a Scene Capture Component 2D in your character attach it to your camera
-Create texture render target and assign it to the scene capture component.
-On a button press call “Capture Scene” on it
-Assign the render target to texture parameter of the material on the plain.

This would work if you only wanted to use the players viewpoint. if you have a 2d hud or userinterface then that would not get captured.