I can only think of one way to sort of do this at the moment.
- Set your base resolution to 2560x1440
- Reset your capture source's size, it should be 3440x1440
- Center the capture source in your scene
The sides of the source will fall outside of the scene, giving you the 2560x1440 center section of your display, which you can then downscale to something close to 1080p. If you can get all of a game's HUD elements to appear in what would be the "center monitor" area instead of the sides then this should look fine for viewers.