EDIT: updated with full solution and correct Unreal version :roll: .
Killing floor only showed a max 1680 x1050 as default but as it uses the Unreal
2 game engine its possible to do a couple of simple changes to the games .ini file to get the desired 5040 x 1050 widescreen res.
Using the instructions on this forum for Unreal 4 and a bit of common sense I can confirm the following worked for me.
The file you need to edit is
killingfloor.ini, which is in the Killingfloor/system/ folder.
Search for the following lines in the file and update them with the new res:
WindowedViewportX=
WindowedViewportY=
FullscreenViewportX=
FullscreenViewportY=
e.g for 5040 x 1050 widescreen:
WindowedViewportX=5040
WindowedViewportY=1050
FullscreenViewportX=5040
FullscreenViewportY=1050
You will also need to change ScaleHUDX=0.000000 to a new value to adjust the HUD.
I found ScaleHUDX=0.333333 fitted my HUD to the centre screen, however it distorts the weapons and timer so I am using ScaleHUDX=0.666666 which uses all 3 screens but weapons display perfectly. This fits quite well with only your weight info being slighly cropped at the bottom of the screen but still readable.
Screenshots
Original 1680 x 1050
Widescreen 5040 x 1050 (using 0.666666 Hud scale)
The game can be purchased on Steam:
http://store.steampowered.com/app/1250/