The following Python script will add all the files given as arguments to the recently-used list, using GIO:
#!/usr/bin/python3
import gi, sys
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, Gio, GLib
rec_mgr = Gtk.RecentManager.get_default()
for arg in sys.argv[1:]:
rec_mgr.add_item(Gio.File.new_for_path(arg).get_uri())
GLib.idle_add(Gtk.main_quit)
Gtk.main()
The last two lines are necessary to start the Gtk event loop; if you don’t do that, the changed signal from the manager won’t be handled, and the files won’t be added to the recently-used list.