I wouldn't think it would be bad, but Epoxy's aren't totally "hard" just as soon as they've cured, I'd let it cure for a WHILE, more than the standard 24-48 hours. It also doesn't bond great to polyethylene or polypropylene in my experience. After it is completely hardened all the way through, it is non-toxic, and I wouldn't imagine it would be continually giving off hydro-carbons or the like. I would however if possible test first (maybe on a small area outside of the mash tun), just to ensure integrity of the product with the type of plastic you are using.
I love the stuff, I use it all the time. I ran out of superglue during a project, had some JB Weld sitting there unused, and I have been keeping it around ever since.
You could go look to, there are a lot of"food-safe" resins, but I think you just want to err on the side of caution extremely when it comes to curing.