Portal support in GTK / GLib

Hi,

i try to use my Formiko application use in Flatpak. So i found may
documentation how can I do, and what can i use to do application in
best practice way, but:

* I want to create backup files before saving, so writing to
  filesystem without GTK Dialogs.
* I need to open other files like images which are linked with
  Forimiko dokuments - in Webkit.

And this is about complete using own file system wrapper, which is not
implemented in GTK portal support now. So, question is:

* Can I disable portal using in Native widgets, like
  FileChooserNative ? Yes, I know, that at this moment, I could
  use FileChooserDialog, but FileChooserButton use Native
  widget now. And what in the future?
* Is planned to do some wrapper about filesystem, like
  FileIOStream in Gio or something like that?

Thanks a lot

PS: Formiko use GTK3 at this moment.