I receive a string that contains valid JSON from another service. I would like to just forward this string with Nancy but also set the content-type to \"application/json\" which
If all routes of your module return a JSON string, then you can set the content type in the After hook for all routes at once:
After
Get["/"] = _ => { // ... return myJsonString; }; After += ctx => { ctx.Response.ContentType = "application/json"; };