How to instantiate the Show class after definition of the data structure

Question

I want to derive a Show instance for a data structure which is very simple:

data MySQLConnectInfo = MySQLConnectInfo
    { mysqlHost       :: String
    , mysqlUser       :: String
    , mysqlPassword   :: String
    , mysqlDatabase   :: String
    , mysqlPort       :: Int
    , mysqlUnixSocket :: String
    , mysqlGroup      :: Maybe String
    }

I do not wish to modify the library's code manually. Is it possible to automatically derive a Show instance well after the definition (in another module) as one would create it when adding a deriving clause after the data definition? I tried to leave a instance Show MySQLConnectInfo but the compiler complains that there is no definition of show or showPrec. It is a pain to manually write an instance of Show, is there a better, automagical way (possibly using GHC extensions)?