Rendering Game with Persistent HUD in Borders

I had a concept for a game on mobile I’d like to try out, but I want the controls in the UI to be persistent on the left and right side of the screen while the game is rendered in the center. Is there a way to improve the performance by just rendering the game at the center of the screen (perhaps by a set aspect ratio) and then completely filling the letterbox on the left and right with UI elements? I’m trying to achieve something similar to Dota 2 where the persistent HUD fills the bottom of the screen while the game is rendered only on the top part of the screen.

Or, better yet, does UE4 stop rendering anything behind opaque pixels in slate/UMG? I am currently under the impression the UI was rendered after the game frame, and I do not want to take a performance hit on the pixels because this concept requires a lot of dynamic lighting.