I guess this is a feature request. Any chance that llgal could be given the option of using thumbnail_create_command to create thumbnails for movies (and not just images)? I feel that updating my wrapper script that I'm already using is likely cleaner than running a separate pass to create custom thumbnails prior to running llgal.
Maybe it could take the form of a image_thumbnail_extensions and movie_thumbnail_extensions that would represent the subsets of image_extensions/movie_extensions for which llgal would attempt to generate thumbnails for. I suppose something similar could happen for scaled images too, for orthogonality (though I personally don't need this bit 😛).