I did have a large hole covered by a grille in an external wall directly opposite the bathroom wall. THis in a mid terrace 1930's house. The bathroom fitter has boarded over and tiled it and will be putting in an extractor fan in the ceiling. He said this was the way to go and I accepted it as he is the professional.
Like everything I do I'm worrying about it as I've now read that it's best to have the extractor opposite the bathroom door on an external wall and only go through the ceiling if you have no choice.