Any advice on how to fix a ding that has gone through a varnish sunburst finish? Is steaming it out a good idea? How about coaxing the wood back level and using superglue to impregnate it?

Hello Ray. Definitely don't superglue the spot.
Although varnish is delicate it can be fixed more easily than laquer by someone who is knowledgeable. Although I appreciate that you don't want to be without your mandolin, this is definitely not a "mr fix-it" job. You'll only devalue your instrument. I would suggest calling Collings and talking to them. Maybe there is a luthier near you who can help. I've seen quite a few old instruments that have been negatively affected by homemade repairs.
Good luck
Doc

Do not use water on varnish! a no, no. Steam will leave white marks. I would leave it alone, enjoy the patina. Problem with varnish is that it is harder to repair, not easier, than lacquer. If you have to repair, take it to a very experienced repair person.

If you can spray varnish and have some touch up colors on hand you can do this. If you dont then i wouldnt try this on your own!!!!!

Step away from the superglue. Forget steaming. Please, I beg you. Call Collings. They'll know what to do. Superglue arrrrrgggghhhhhhhh! NOOOOOOOOOOOOO!