Like Building a Ship in a Bottle
Two days and three trips to Home Depot later, the rail is back in the wall and completely fixed - and I didn't have to punch any holes in the drywall/plaster!!! I've not rehung the panel doors since I want the termite guys to be able to see into the wall cavity. And I also want to give the other door some time to unwarp (its laying flat on the ground with the good door on top, pressing it flat).
Fortunately, the destroyed header was not anchored inside the wall cavity (or the termites ate the anchors out). I was able to remove it completely.
Here is a shot of the old plate, now anchored to two cross-headers that sit on 4 x 4's that were on either side of the destroyed header. These parallel headers are actually beefier than the original one so I'm not worried about their ability to handle the load.
Here is a three foot section of threaded rod attached to the iron rail. I drilled a 4 inch hole in the attic over the void in the wall that I poked the rod through. It went in suprisingly easy given that the rod was 4 feet into the wall and the rail weighs a good 40 pounds. The hardest part was getting all the measurements right for drilling the hole.
Here's the second section of threaded rod joined to the first and then anchored on two cross headers in the attic. The end of the rail now hangs on the rod instead of being slid into a hook hanging from a header. Once the door is hung, there will be a bit of fiddling to get everything level and centered. Then I can toenail in the cross-joists to prevent things from moving. The monster joist behind is one of the two that takes the load of the great room's ceiling since the center wall was removed to make it one big room.
Finally, a shot of the rail in the door frame. It's really solid and I can't wait to put the door back in after the termite guys look at things on Wednesday. I just hope the other door has straightened out.