Nonfree distributions findable in osinfo-db via GNOME Boxes
A Trisquel user, who does not feel comfortable with English, detailed me how nonfree distributions are findable in GNOME Boxes. That program indeed includes a button (at the bottom of the window to create a virtual machine) that leads to a window with a field to search for an OS. There, if one types for instance Manjaro, ISOs of Manjaro are listed. A click on one of them and GNOME Box suggests the creation of a virtual machine running Manjaro. Another click and Manjaro indeed runs.
I guess it is a borderline (because the user needs to search the nonfree software) violation of the Free System Distribution Guidelines: "Programs in the system should not suggest installing nonfree plugins, documentation, and so on." https://www.gnu.org/distros/free-system-distribution-guidelines.html