Which a filly could run against the males in the prep races but it's going to be quite rare. There's quite a few prep races , granted some of the races have lower points then the others. The Champagne, Holy Bull, withers and other races have less points then Santa Anita Derby, Florida Derby, Wood memorial, and others. But it can be done... just not highly likely.
With that being said the last filly to run in the Kentucky Derby was 2010 which was before the points system. So I agree it's going to be much harder for a filly to get in.
I was just informing the OP the fillies can run in the races.
Chad Barnes 6-16-85~7-22-13
Hero Act - Thoroughbred Gelding ~ Gunner - Quarter Horse Gelding ~ John Deere - Mini Gelding