Adding a watermark to a custom thumbnail

Is this possible?

I don't have ffmpeg installed (shared hosting) so I always have to add custom thumbnails for my video files. However, it's impossible to distinguish between a video and an image once I have customised the thumbnail. Is there any way I can get Gallery to automatically stick a film reel watermark on the thumbnail? I noticed that the functionality was added with ffmpeg in Gallery 2.1

# New option in ffmpeg module to watermark thumbnails for movie items with a film reel image on each side, to better distingish movies from photos.

I just need that for the custom thumbnails. Possible?