things you should investigate:
- Use world coordinates to map texture to world pixels
- you can also do it like skyboxes are done, calculate reflection vector use it to map pixels from texture, you can mix this with above method.
- you can attach 2d plane ho each player, and move it ro where camera is looking at, also map texture to world coordinates.
- there was also postproces way, however not sure if it is still easy to detect where is background and where are level actors. In 4.12 it was quite easy so one could swap whole background with about anything
picture of actual game setup in level would help to narrow down what to do.