I am not sure why you would want to use a magnet in the hop bag. 304 stainless steel is not magnetic so it you are trying to get the magnet to stick to the bottom of the mega pot to hold the bag down in the beer, it ain't gonna happen. Secondly, magnets are usually made from rare earth materials and I am not sure I would want to take the chance of anything from the magnet leeching into the beer.
I assume you are using pellets for dry hopping? If you want to weight the bag down, see if you can find a stainless steel weight to do so. The stainless steel balls that are used in spears of Sankey kegs work well but finding them might be a pain, unless you know someone who has cut the top off of a keg (obtained legally) to make a keggle. You may be able to find something else that is made from 304 stainless that will do the job. Just be aware that some stainless steel will rust, so you want to make sure that you use 304. As an afterthought, you might contact SABCO in Toledo, Ohio and see if they will sell you some of the stainless steel balls they use in Sankey keg spears.