I live in Villach, so only a couple of hours south of Salzburg. I only have one September under my belt but I know that September can be changeable weatherwise. Although last year I had fantastic weather at least 1 day each weekend until October.
I would definitely head over to Saalbach. The lift system is open until the end of September and there are trails for all abilities. I was there the weekend before last and rode most of the trails. For blue type trails I would go for the Milka Line and the Blue Line. For Red lines I the Z-line was great (things like wall rides can be avoided with ease), the Hacklberg trail was longer and fantastic, and the Panorama trail. I also rode the X-line which was fantastic but above Red level. I did not ride much over nearer Leongang.
Other than Saalbach I can tell you about more riding in Carinthia but not near Salzburg.