Not familiar with a "passenger tail panel seam" so excuse my ignorance. But looking at it, the metal is too porous to attempt to fill it in with additional metal via the TIG. The professional way would be to cut out the rotted metal and replace it with new metal.
Anything else you do to "fix" the rot may not be long lasting. Rust has a way of continuing and trying to cover it up to look good may only hide it for the short term.
If I were to do a quick fix, I would give the area a good sandblast with some of that walnut media which is said not to warp metal (have never used it myself, so no confirmation on this).
Then use a rust neutralizer over the area to saturate any rust still in the metal.
Then use a fiberglass cloth/resin to lay over the area. It will follow the shape as you simply lay it over the damaged area. Let it set up. Then sand the fiberglass to scuff up the surface to make it rough. Use a product called "Kitty Hair" http://www.evercoat.com/product-deta...art/100868/us/
to go over the fiberglass cloth like bondo, and work it as you would body filler to get a smooth even finish. This "fix" should be fairly rugged, be easy to work with, and last long.
Here is a quick basic You Tube video on the stuff. You can use it as the video shows, but I would still use the fiberglass cloth as a backer over the rust holes and then apply the Kitty Hair. https://www.youtube.com/watch?v=an8WcfTtLog