To everyone who is having / has had horrible back pain caused by the Prius' driver seat: I have a partial solution that involves padding the driver seat with foam blocks and wedges.

I discovered that, despite the nicely curved contour of the driver seat, the actual foam inside the seat does NOT follow the contour of the fabric. Instead, the fabric is stretched so tightly that it just looks like foam is there underneath it, as one would expect. Instead, it is just air, and the foam itself is much further below (I use "much" as a relative term). Clearly, they want us to provide our own foam to complete our seats, which they can do because they've already deceived us into buying the car.

I have been doing the following for the past several weeks, and it has made a noticeable improvement in how my back feels after driving the Prius (this is after several years of horrible back pain from the anti-ergonomically designed Prius driver seat):

One of the most important pieces of foam is a foam pad, to be placed on the seat. I found the foam used in the Prius seats to be very cheap (examining the actual foam itself, the air pockets are not uniform) and uncomfortably hard; but put a nice, high-quality foam pad on the seat and you will feel much better. Note that if you are tall, you'll probably want to get a thinner foam pad; otherwise, the foam pad can be as thick as you need to raise you up high enough.

The second most important piece of foam is a wedge to support your upper spine. Something just wide enough for your spine, and thicker at the top than at the bottom; this should push your upper body forward about an inch, so that the headrest is not causing your head to jut forward as much.

If you are not incredibly long-legged, then you'll find, like me, that the car seat is much too deep to be comfortable. Place enough short foam blocks at the base of the seat to push you forward; you'll probably want your knees to be past the edge of the seat so that you can bend them. The foam blocks should only be as wide as the inner part of the driver seat - if they're too wide, they'll start to curve. The foam blocks should be just tall enough to support the next item on the list:

Two foam wedges for lumbar support. You should place the first wedge so that its thick part is lower than its thin part. The second wedge goes in the same direction, but slightly lower, on top of the first wedge. The idea is to provide support to your lower back.

The foam I used for everything is a nice, soft but supportive, high-quality foam - all bought from a place called House of Foam.

Using this configuration helped my back greatly after only a few days. It's been several weeks now, and I can finally carry things on my back again.

The only thing I can't fix with this configuration is the headrest pushing the head too far forward - the metal bars in the headrest actually are part of a "U" that is also bent in the wrong direction (namely, forward). The foam is needed so you don't crack your skull against the metal U-bar when you put your head back or in an accident. If anyone has a solution to this problem, or any improvements that they've tried and are satisfied with, please post them here - thanks!