OK, finally found it, although the installation path is neither linked nor mentioned anywhere (that I could find) in SneakyTweaker. It installs mods at C:\ProgramData\Thief 3 Sneaky Upgrade. Never would have thought to look in ProgramData, since 99% of programs use C:\Users\(username)\AppData\. Having the Garrett Animation Tweaks installed is why changes to T3PlayerAnims.ini in the game's base folder was having no effect. Thanks snobel!
I've been trying to install my old Dark Project disc version of the game with Tfix Lite for a close to vanilla experience. A week or so ago I got the old game installed, but I inadvertently installed Tfix instead of Tfix Lite. So I uninstalled and have been trying to reinstall today, with no luck. I get as far as the install screen, click 'Install', and after about a minute the screen will disappear and nothing will seem to be happening. Any ideas?
EDIT: Hey, they weren't kidding when they said it might take hours for installation to start. Installation successful, game runs, and Tfix Lite is, I assume, applied successfully (I have more resolution options than in vanilla).