Thanks to Paddy the Wak for the solution.
Original TH2Go thread.
Most widescreen resolutions are supported natively. Check to see if your is. If so, just select it and your aspect ratio in the ingame options screen. But some resolutions, such as 1280x768, are not supported natively, and must be enabled through editing a config file. If yours isn't supported natively, then follow this solution.
Go to My DocumentsEnemy TerritoryEnemy Territory Quake Wars Public Betabase.
Edit etqwconfig.cfg. Find these lines:
seta r_customHeight "xxx"
seta r_customWidth "xxxx"
Change them to set your resolution. Remember not to get the height and width mixed up.
Next, find this line:
seta r_mode "x"
Set it to "-1".
If your resolution is 16:10 or 16:9, then just start the game and select your aspect ratio from the ingame options screen, and that's all you need to do. If it's something else, then you need to do a little math.
Write your resolution as an improper fraction, with the horizontal number on top and the vertical number on the bottom. 1280x768 would be 1280/768. Now simplify the fraction. 1280x768 would become 5/3. Once it's simplified, the number on top is your H-value, and the number on the bottom is your V-value.
Find this line:
seta r_customAspectRatioV "x"
Set it to your V-value. Then find this line:
seta r_customAspectRatioH "X"
Set it to your H-value. Then find this line:
seta r_aspectRatio "x"
Set it to "-1".