Actually I was thinking of adding a graphic symbol just like the rehearsal marks but for the outro. Not necessarily having the player to process it.
You're right, the player can use the coda sign to play a section at the end of the song, but it doesn't work when the intro is also the outro unless we re-write the intro at the end of the song with a coda sign. In some cases there are long intro/outro (16 bars or more) and it's not always possible to write it twice because of lack of space.