There are some heat barrier products that are designed to prevent heat from traveling deeply into a panel. For instance, there is a paste-like substance that can be applied to the metal, and it actually does limit the heat transfer where it contacts the metal. Unfortunately, it's hard to get this material into the areas where it is needed most, since you have to have a clear shot at the area to be welded, so you can get your welding torch or gun into position to make a weld with no obstruction. If the heat barrier material is out of the HAZ, it's not really doing you much good.
Some people have told me they first put a strip of masking tape on top of the joint to be welded, then "butter up" an area a couple of inches on either side of the joint. Then they pull the tape off, leaving a clean strip of metal, centered on the joint, while still having a liberal application of the heat barrier pretty close to the weld. I haven't experimented with this process, but I'd expect a significant reduction of warping with gas welding, although this would prevent you from hammering the weld when hot. With TIG welding, I'd expect only a small reduction of warping, and probably not much reduction of warping at all with MIG welding, because the HAZ is so small to start with.
Now you can e-mail your questions to Professor Hammer at covell@cruzio.com or send mail to Professor Hammer c/o STREET RODDER, 774 S. Placentia Ave., Placentia, CA 92870. We'll print your name and city unless you request otherwise. Ron Covell has made several metalworking videos, and offers an ongoing series of workshops across the nation covering all aspects of metalworking. Check them out online at www.covell.biz, or call (800) 747-4631, or (831) 768-0705 for a current schedule of workshops, or for a free catalog of videos, books, and fine-quality metalworking tools. You can also send a request by mail to Covell Creative Metalworking, 106 Airport Blvd., #105, Freedom, CA 95019.