The Gtk+ file chooser supports the addition of arbitrary widgets, which
are shown below the file list. One could use a toggle button to make the
"Show hidden files" feature visible:
Since it is possible to add a "Show hidden files" toggle button to
the file chooser, is it possible to do the same for the filename
entry field from the small window that pop-ups after C-l?
This would be consistent with the default Emacs filename reading behavior,
i.e. the minibuffer in Emacs is like the filename entry field in the
file chooser, and the *Completions* buffer above it is like a list
of files in the file chooser.