As mentioned on the main page a custom resolution can be specified by editing the config.ini file, but trying to play the game afterward will result in a crash after the main menu.
The crash seems to be caused by game trying to look up the resolution in a table of known resolutions in the EXE (the list displayed in game settings), if it's not found there then game will crash.
Getting around the crash is a simple matter of just editing one of the existing resolutions in the list (eg. 2560x1600) to the desired resolution, that combined with INI edit seems to allow game to play fine without crashing.
Unfortunately even though the resolution list can be found unencrypted in the SteamStub'd rerev.exe file, editing that will then cause SteamStub to refuse to launch. (I think there might be a flag that can be changed in the .bind section to skip verifying EXE contents, but not sure about that)
I just used Steamless to remove the SteamStub layer first, and then the EXE can be edited fine without issue. (note that launching EXE directly doesn't seem to work, has to be ran through the Steam launcher)
I don't know the policy about recommending tools like Steamless in main articles though, so haven't added it there. Maybe asking users to edit both EXE+INI along with using Steamless tool is a bit much too... maybe I'll make a small DLL hook to edit it in memory soon.