As the now much more experienced OP, I'll respond to my own thread

One thing you might try if you're really committed to renting from National is seeing if you can match at the counter when you arrive. I haven't tried it very often, but I *have* found that occasionally walk-up rates are better than anything in the reservation system -- and in at least one case, I was able to secure a car when the 800 number said none was available.