Is it safe to use staples or paper clips to seal parchment paper when cooking fish en papillote?
As long as they are not plastic coated, it is unlikely for anything relevantly toxic to get into the food via vapors just from heating a small household metal object in the same space with the food, below temperatures that will cause anything to melt. That would require some constituent having so high a vapor pressure at 450°F that a lot of it would go into the air... 

Unknown metal objects in contact with the food, especially if they get immersed into an acidic sauce, could be more of a concern since there could be reactions with the metal and metal salts entering the food - though it is not that likely for toxic elements to be present in the steels or other metal alloys used for objects like paperclips or staples. Lead (from some brass alloys, or from some kinds of bright tin finish), aluminium (usually not added to steels), cadmium or zinc plating (don't use anything decades old and looking yellowish. This can be a type of zinc coating, but it could also be cadmium plating. zinc (which also is found as a non-yellow plating) is not per se toxic in small amounts, but FDA rules seem to discourage reactive foods contacting zinc-galvanized steel), copper, pure chromium or nickel (these are used in food-safe steels too, I would not be so sure they are food safe when used pure as a plating) are probably the alloy elements to worry most about...