Are there some tools that make it easy to make these filters? It would be nice to have something built-in (as a plugin?) to mpv or VLC.

I've created several filters for personal use with EDL. mplayer has a decent UI way of doing that while watching the film by pressing 'i' to start a cut and 'i' again to end the cut. It's nowhere near perfect though because you still have to edit the edl file to decide whether to cut video or mute audio.

One nice feature you could add to this editing UI would be to use the subtitle file to allow jumping to the locations of certain language in order to edit them (times in subtitles aren't enough to edit a single word).