Tried using JsonSerializer.DeserializeFromString<ExpandoObject>(data) and it does not work. Can one deserialize into dynamic, or is SS not capable of doing that?
It's not supported by ServiceStack serializer. You may get it working to a degree if you were to compile the serializer in .NET 4.0 like suggested by Dennis. NuGet package only contains .NET 3.5 build. I think it's not worth the trouble. Best would be to use JSON.NET in this case. Here's an example.