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!