A co tohle?

Run both the singleplayer and multiplayer executables at least once, to generate config files. Go to <gameDir>\GameData\base.
Open jk2config.cfg and edit these lines:

seta r_mode "x"
Set this to -1.

seta r_customwidth "xxxx"
seta r_customheight "xxx"
Set this to your favorite widescreen resolution.

Add this line to the bottom of the file:

seta cg_fov "xx"

I recommend 90 for 16:10 resolutions, and 95 for 16:9 resolutions (default is 80).

Now do the same for jk2mpconfig.cfg.

Single player anomalies

There is an annoying fault - when widescreen mode is enabled, the loading screens make your monitor display a solid white screen.

Supported Resolutions
16:10 Resolutions
1280x800
1440x900
1680x1050
1920x1200
15:9 Resolutions
1280x768
16:9 Resolutions
1280x720
1920x1080
Eyefinity / Surround Resolutions