Comment by em-bee

a startingpoint for answers may be here: https://superuser.com/questions/944119/replace-gtk-file-dial...

there is an xdg-desktop-portal feature that allows this, or pretend that a different desktop is used by setting XDG_CURRENT_DESKTOP for the application, as well as KGtk which is a library that can be loaded with LD_PRELOAD.

either of these have to be applied per application though.