Are there any GUI applications (linux or windows) that allow you to apply ffpmeg filters to a video and observe the changes in a preview in real time?
FfmpegGUI seems what you want: https://github.com/RomanSichkaruk/FFmpegGUI
The Avanti GUI, for Windows, allows you to preview filters. One has to select the FFmpeg Script Editor tab and insert
-vfout filtername under
[USER_VIDEO_OPTIONS]. Consult the docs for exact syntax. Once entered, click on the 'Preview Filters' button.