Many members of our club (TSG) expressed an interest in a Croesor trip, followed by an afternoon in there. Personally, I think it's a good use of an empty space, as long as it's safe.
On a similar level, I went into an old bauxite mine in Les Baux-de-Provence a few years ago, which has been converted into an 'audio-visual presentation' venue, utilising slide and overhead projectors to plaster the walls with images - our presentation was about Impressionist painters, and was very impressive, as the walls of the stopes were totally flat, making the projections of very high quality. Here's the site:
http://www.carrieres-lumieres.com/en/home