The stock brackets are welded to the seat sliders, so your only real option is to change the sliders, and possibly the whole assembly.
I did some searching and it looks like Toyota Echo seats have the same mounting points (they share the same chassis as the xB), and the seats sit lower in the car.
Compare the xB seat bracket rear with the same place on the Echo.
As you can see the xB has a "riser" that brings the seat up roughly 2". So if you can find a set of Echo seats you can probably combine the piece from those seat tracks with your seat tracks to get yours lower. Plus as a bonus the Echo seats should come with the storage tray under the passenger seat.
If that isn't low enough for you then you're probably looking at custom modifying both the brackets and the floor due to the floor brace the front of the seat bolts to.