The list of files that such app would try to access by default on initial startup would be finite and well defined. So, a Flatpak package of such app should alr

The list of files that such app would try to access by default on initial startup would be finite and well defined. So, a Flatpak package of such app should already, if done "properly", (I'm just imagining here, not an expert) include allowed access to those paths by default.