Being able to copy / paste file paths is a feature that should probably exist, I'd imagine that would be accepted as a feature request.
It won't happen in GTK3, that's feature frozen. Maybe someone could implement it in GTK4?
I talked to mclasen on GIMPnet IRC #gtk a month or two ago and he did fix this in Gtk4. You can paste without typing anything beforehand in Gtk4 file chooser.
Additionally, the old type-ahead complete was removed from filename-entry. Arch at least has a patch set to put this back, but the location-mode thing is resistant to fixing. I changed the default initialization with a simple patch to gtkfilechooserwidget.c but it only works for the first time I File->Open in a gtk3 application. The second reverts back to pathbar.