You can physically swap out any Content Type Serializer by registering a Custom Format, e.g:
appHost.ContentTypes.Register(MimeTypes.Json,
SerializeToStream,
DeserializeFromStream);
Which is what all the Custom Formats use:
Just don’t expect any of the JSON customizations or all Add ServiceStack Reference supported language data types to work as they expect features and conventions in ServiceStack.Text.
But if you’re just serializing a generic JSON API, e.g. to ajax clients it should be ok.