Meld uses the gtk.FileChooser class without set the set_local_only property value.

Unfortunately the default value of set_local_only property is True.

With this default value, paths like gvfs mounts (eg. sftp://host/file.ext) are not allowed.

I tried to add these 2 new lines and the remote paths works well: 

File: meld/newdifftab.py

         for chooser in self.file_chooser:
             chooser.set_current_folder(default_path)
>>>          chooser.set_local_only(False)

...

            for chooser in self.file_chooser:
                if not chooser.get_filename():
                    chooser.set_current_folder(parent)
>>>                 chooser.set_local_only(False)