I made very small progress: If you analyse Zeus.inf (which is the .inf mentioned above) in a Hex editor, you can see that at 0x10 is a value telling the game which of the resolutions was used last time, i.e. your settings. 02 for 800x600, 03 for 1024x768. (I'm guessing it will be 01 for window mode). This shows that the .inf is actually not useful in customizing a resolution. The actual resolution values are stored somewhere else, maybe in Zeus.exe itself.
EDIT: I made somewhat of a progress... I managed to change the fullscreen resolution from 800x600 to 1280x720 by editing Zeus.exe, but the game itself won't scale accordingly. It looks totally messed up:
(German version, incl. Poseidon add-on in case you are wondering)
If it is helpful for anyone: I searched for 58 02 and checked if there was a 20 03 nearby (800 and 600). Everytime I found something, I changed it to d0 02 and 00 05 for 1280 x 720, saved and started the game to see if anything had changed. The resolution could be changed at 0x17ccb1 and 0x17ccbb. (
20 03 00 00 c7 05 e8 68 fc 00
58 02 00 00). Any suggestions what I could try?