I powder coat gas tanks all the time. Have done Speedway and Rupp mostly. But also Gemini and Broncco tanks too.
Yes they are soldered. and yes the solder does melt at 400 degrees. But you can take some precautions and it's not a problem. Before i was doing my own powder coating, i would have my "regular PC guy" do tanks all the time, with no special work on his part.
what I do is TIG tack weld soldered parts on the tank. Anything that's soldered (aside from the center seam), gets a TIG weld tack. That includes the mounting bracket, the 1/4 NPT bung plate, and the filler neck. Also always powder coat the tank upside down (bung up). Doing this i have had no problem with powdercoating gas tanks. I've probably done no less than 20 Rupp tanks and 20 Speedway tanks, and some other brands here and there.
Another trick is to make sure the tank (upside down) is level. This way when the solder melts, it won't run off. It just pools. Then after the oven bake, let it cool level too. The solder will solidify and all is good. And even if the solder does run, usually it just flakes off after cooling without any damage to the powder.
As for baking the powder at lower temps... If you can convince your PC guy to bake at 350 to 375 that is helpful. But frankly they nearly always use 400 degree, whether you ask or not for a lower temp. So when i was sending out gas tanks for powdercoat, I stopped asking for the lower temp.
Now realize i usually seal the tanks after powder coating. But not so much because of the powder coat. Mostly because they are just old gas tank and nearly always have rust inside. So when i'm sandblasting the outside, I usually blast the inside too. Then I use POR-15 after the powder coating. It's a safety precaution I take so there aren't any problems. But frankly I've powder coated a ton of Rupp tanks without sealing them.
For example, all these Speedways I just powder coated, including the gas tanks. And these are "illusion" colors, which means that they are powder coated *twice* (once for the color, and once for the clear.)
http://www.pinrepair.com/minibikes/p/speedway_frames_pc1.jpg
I should also note that the Speedway gas tanks which were powdercoated in the above picture link had significant dents. They were filled with JB Weld, and then sanded to shape. You can powdercoat JB weld (at least the 24hr dry version, I have not tried the 5 minute JB weld.) The trick though is to pre-heat the gas tank before powdercoating it. I put the tank in the oven and pre-heat it to about 350 degrees, and then powdercoat it while it's still hot. This works really well if you have tank dents that need to be filled. You can powder coat JB weld cold too. But I've had some colors that this did not work well. So pre-heating the tank before powder coating seems to be a better idea.
