I've never tried this because I don't have a solid aluminum dish, but I wonder if one or more strategically placed reptile tank heaters would work, just be sure you get one(s) that can be attached to metal. Typically they are peel and stick, so as long as the aluminum surface is cleaned and dried well first, I'd think they would stick. They are meant to be placed under a glass tank and provide warmth for cold-blooded critters, and they usually don't draw much current, depending on the size you get (and they do come in various sizes). The only reason I think this might possibly work better than heat tape is because of the peel and stick arrangement, but that said I don't know how well they would conform to the concave surface of a dish, since they are meant to be applied to flat glass. Nor do I know for certain that they would generate enough warmth to be conducted through the aluminum for any distance (aluminum is used for computer heat sinks, so it must be somewhat conductive of heat).
Maybe get one unit and put it near the lowest point (about halfway between the lowest point and the center) on the back of the dish and see how it does. If it stays attached and melts a significant amount of snow, you could always get a couple more and place them nearer the sides of the dish for better coverage.
If you do get heat tape, I advise against the flat ribbon vinyl plastic crap, it tends to stop working after a season or two. The better stuff is a metal shielded cable that can be cut to the required length, and lasts a very long time, Easy Heat is one brand of that type and some hardware stores will sell it by the foot. Maybe you could attach it to the dish with some of that foil tape used to seal heating and dryer ducts/vents? I haven't had the need to buy any heat tape in a very long time but I do remember what lousy crap the flat vinyl ribbon stuff was. I would not be at all surprised to find out that stuff has been banned, since I think it may have been responsible for a few mobile home fires.
