Many thanks to Frederic Peters, GNOME Scan ref doc is now at library.gnome.org :  http://library.gnome.org/devel/gnome-scan/0.6/. If you fill adventurous, you could try writing a plugin for you prefered app or write a standalone app on top of GNOME Scan. Beware that API is supposed to change for 0.8, in order to provide bindings (Gobject introspection and vala first, then python and more).