@ FrostWarrior: I’m not really sure whether I understand the issue. If you created a custom resolution of, let’s say 2240×1680, and select this resolution in the Half-Life 2 video options, you are set.
All you need to do is making the game run with your added custom resolution. In case of Half-Life 2 that has to be done in the game options.
In other supported games you can do do it from the vorpX menu, which is what moadepht is refering to. Doesn’t work for HL2 though.