OK, I found it. Very deeply hidden,: if, from the extended options, zoom to 1,399 is selected, and from Video -> filters -> crop (second appearance) crop aspect ratio to 16:9 is selected, the letterboxed movies are displayed without black bars on a 16:9 display. Great!
I would surely love to see a menu entry like "crop to 16:9" doing those settings, maybe on the fly without having to stop the movie.
Anyway: great piece of software!