I don't think any of the fillers with metal in, body fillers like Upol D or chemical metals like plastic padding will stand the temp, not that I have tried them, but I have used JB weld and it is fine.
You can buy fillers intended for coating over, two names I can think of are Lab Metal and Themobond, I see them listed on the products pages of coating suppliers. The powdercoater might well have something like that.
I will try lead free solder when I run out of JB weld, lead free melts at a higher temp than leaded solder.