I should note that I used the Heretic 2 demo, because I don't have the full game. Let me know if the solution works for you and we can try to get it added to the game list here at WSGF.
Being a Quake engine game, you'd figure adding widescreen support would be a trivial matter, but I was unable to find an ini solution using the Quake config parameters, unfortunately.
I was able to add widescreen support by hex editing the Quake2.dll file found in the Heretic 2 directory, though. If that file doesn't exist in the full version, you may find the equivalent file named Heretic2.dll or something.
You'll have to use a hex editor such as Be.HexEditor. Use the program to open said .dll file (located in the /Heretic 2/ directory), and search for the string highlighted below.
00 05 and c0 03 represent the resolution 1280x960. What we'll do is change them to represent the widescreen resolution we want.
1280x720 - Replace 00 05 with 00 05 and replace c0 03 with D0 02
1280x800 - Replace 00 05 with 00 05 and replace c0 03 with 20 03
1366x768 - Replace 00 05 with 56 05 and replace c0 03 with 00 03
1440x900 - Replace 00 05 with A0 05 and replace c0 03 with 84 03
1680x1050 - Replace 00 05 with 90 06 and replace c0 03 with 1A 04
1920x1080 - Replace 00 05 with 80 07 and replace c0 03 with 38 04
After you're done, save the file and close the hex editor.
This is where things can get a bit hairy. Simply loading up the game and choosing OpenGL - 1280x960 wouldn't really work for me for whatever reason. I was able to get it working by selecting the D3DGL renderer (in the Video Options menu) and selecting 1280x960, though. It then set me to the right resolution, and then I was able to choose the OpenGL renderer.
From there it ran without a hitch. You will want to change the fov either through the console or through the config file located in the /Heretic 2/user/ directory. Just open config.cfg and locate the fov line, which should be at 75 by default. The correct FOV for 16:10 resolutions is 85.2812718522, so use that instead.
1024x768 (4:3)
1440x900 (16:10)
