True, your back is healing. On the other hand even a small head-on collision will fracture your neck, it's very uncomfortable, the ceiling is exceedingly dull, and the dog is smelly:
Clearly, inspiration is needed.
Experimentation shows that any time spent in a conventional car seat is a painful trip to acute muscle spasm. On the other hand, maintenance of exaggerated lumbar lordosis (curvature of lower back) and neck extension is well tolerated.
How can one transform a conventional bucket car seat into something that will support an upright posture for 1,500 miles of driving over about 8 hours a day for two days?
Well, it has been done. In fact, while sitting in my custom rig I healed more quickly than when lying flat on the rather uncomfortable van floor.
The total cost for the rig would be about $25. Since I already had the freezer insert and the neoprene wrap it cost me about $8 for the Walmart mini-boogie board . (See photo below, it's about 24" tall.)
- I adjusted the seat so that it was as close to a right angle as possible, with the base as flat as possible.
- I placed the foam boogie board along the seat back to create a non-yielding seat back.
- I removed the seat's neck protector as I found I needed more neck extension than the seat headrest allowed 
- I inserted the 1" thick red hard plastic frozen picnic cooler  into the neoprene wrap and fitted it so the plastic spacer was either in the middle of my lumbar curve or just below the curve. This created a fixed exaggerated super-cooled  lumbar curve.
- Every 2-3 hours we stopped the car. I walked until I'd loosened up, then attached my inline skates (I figured out how to do this while keeping my lordosis extended) and skated for 10-15 minutes with Kateva to keep the back as limber as possible.
When we finally arrived home, I exited without pain. I even fell a step at the neighbors and my back survived.
Next week I have my first appointment for back care in over 27 years of 1-2 times/year severe disabling acute back pain. It's time for me to get a serious lifelong exercise program in place so I don't have to do this one again.
I will be using my custom back support for a few days however, and when we do our next car trip I'll put the boogie board somewhere. The kids can use it in the pool anyway ...
 This is medically illegal. It will cause you to develop frostbite, skin and muscle necrosis, toxic shock and you will die. You should use a non-frozen item of similar size and shape.
 This will cause you to fracture your neck in a car accident. You will be paralyzed and on a ventilator, then you will die a slow death that will bankrupt your family.
 I wandered the aisles waiting for inspiration to strike, looking for something that would provide firm but lightweight back support. High impact styrofoam in a fabric cover was perfect. I just happened to set eyes on the mini-boogie board ...
PS. There's a bit of irony here. I first wrecked my back 27 years ago boogie-boarding with a full-sized board in Southern California. Maybe that's why I really chose this fix.
Update 7/7/08: I received a post skeptical of my enthusiasm for cold therapy vs. hot packs. I followed up with my new team at Physicians Neck and Back Clinics (profiled in the New Yorker in April 2002, they represent the "new wave" approach of aggressive exercise based rehab). They never use heat or hot packs. They use only cold therapy, though for them strength and flexibility are 90% of the solution. I suspect hot packs can be very helpful for some, but they're really out of fashion. I've only ever bothered with cryotherapy.
Update 3/4/09: I'm going to write a bit more on this topic in July of 2009, when I'm a year post event. I've done well for long enough though that I'm a cautious fan of the extreme core strengthening approach of Physicians Neck and Back Clinics. It might be overkill, but I wouldn't be surprised if it turns out to be about the right balance.
Update 8/1/10: I did the f/u post in Nov 2009, but forgot to backlink it here.