enamel repair for cast-iron cookware?
I'm not sure there's a way to do this. Enamel is glass. I think from what I remember of the process, it's put in in a solid molten form over hot cast iron. Not sure of the temperatures of all this but I don't think it's something you can patch.
That being said, I cook lots of things with tomatoes in the gravy or sauce in my well-seasoned cast iron dutch oven, deep skillets, and pans. There isn't any problem other than they need a little bit of oiling after that and I never let anything sit in them after it's finished cooking. The chip may not be much a problem.