- you could build a non-real-time version of the game engine and use the neural net as a real-time approximation
- you could edit videos shot in real life to have huds or whatever and train the neural net to simulate reality rather than doom. (this paper used 900 million frames which i think is about a year of video if it's 30fps, but maybe algorithmic improvements can cut the training requirements down) and a year of video isn't actually all that much—like, maybe you could recruit 500 people to play paintball while wearing gopro cameras with accelerometers and gyros on their heads and paintball guns, so that you could get a year of video in a weekend?