So, I've written the program. The hack now only requires a few clicks of the mouse.
The solution still isn't perfect. In addition to the must-restart-to-change-FOV problem, there are two other niggles:[list][*]It seems BaseFOV doesn't affect the bow zoom. So, zoomed bows are still vert-, I think.[/*:m]
[*]This almost certainly only affects the original game, meaning fan missions will still play in vert-.[/*:m][/list:u]But it's still a step forward for those of us who just want to do first-person thieving, and means the game can now be considered fully hor+ except for fan missions.
Step-by-step instructions for Thief 3 hor+:[list=1][*]Set Resolution to -1 in options.ini, as in the vert- solution.[/*:m][*]Run T3FovPatch.exe. Select the aspect ratio you want, and click "Change FOV".[/*:m][*]Start a new game and enjoy.[/*:m][/list:o]
Edit: remove old version download link. See
this page for the fixed version download.