UPDATE Found the issue -- was inheriting from wrong class, needed to be JsonConverter.
I have a class that has a Location property of type System.Da
Turns out I just needed to inherit from JsonConverter instead of CustomCreationConverter, and everything else I was trying to change was OK all along.
I'm still not sure if there's a way to apply the JsonConverter fluently, but there is another way to apply the JsonConverter without referencing Json.NET in your domain/core project or marking up your domain classes with references to a peripheral library:
var jsonSerializer = new JsonSerializer();
jsonSerializer.Converters.Add(new DbGeographyConverter());
jsonSerializer.Serialize(jsonWriter, place);