If I'm envisioning your setup correctly, I think a weld, rivet, or other hard fastener is the only way you'll get long-term reliability from the repair. There might be some adhesives that could hold the broken bracket at the temps you describe, but my guess is, if the bracket is under some load holding the lid down, the adhesive would drift under that load and at that temp.
That said, I'm not sure there's anything food-grade that exists, at least from an FDA/USDA standpoint. Most of the food-grade adhesives I've dealt with usually don't see more than 200-300 F, and even them I'm wary of using them since, over a period of time, heat-ups, cool-downs, and sanitation cycles will invariably cause the adhesive to break down, and potentially contaminate any food source you have in its vicinity.
For a home roaster, you probably don't need to adhere to such stringent standards, but it's likely that a polymer-based adhesive would give off fumes as you suggest. My $0.02.
