I think this will work, note the Stream as return argument.
[WebGet(UriTemplate = "{id}", BodyStyle = WebMessageBodyStyle.Bare)]
public Stream Get(string id)
{
OutgoingWebResponseContext context = WebOperationContext.Current.OutgoingResponse;
context.ContentType = "text/plain";
return new System.IO.MemoryStream(ASCIIEncoding.Default.GetBytes("blah"));
}