Once again, we need to keep in mind the intended usage of the guy who started the thread. Rocker bars might be fairly useless in a collision with another car. If the intent is to push your luck in the canyons, there are other considerations that rocker bars might be of some help in dealing with. I believe tube80z convinced Dave Kipperman to put the rocker bars in his car after seeing a hillclimb car go over the edge. In that case the guy's cage managed pretty well, but the driver's feet were smashed because the cage stopped at the front of the door. The intended usage is similar. And they will stiffen the chassis, as the rockers are the main structure that Nissan used to tie the front and rear of the car together. The rockers are more structural than the A and B pillars, or the sheet metal "frame rails" which don't even connect front to back in the stock configuration. Personally, you couldn't get me to push to 10/10ths in a canyon without a heck of a lot more than rocker reinforcements, but I think that point has already been made so I'll leave it at that.
As far as DOT approved goes, is that a legal requirement for seatbelts? If it is, my first inclination would be to adapt a seatbelt out of a newer car. I had a friend with a 510 who got some belts out of a vanagon if I remember correctly and adapted them in. The thing that he was after was a retractor which didn't bolt into a recessed area of the body.