I've just noticed now, in Phil's reply from 6 years ago about a trip of his from 1968 where a couple of the party visited the top stope, but nothing of any remark is noted.
Still, from a mining history point of view, a trip up there in order to determine the working methods and conditions would be a worthwhile project.
I can't think of any other similar, apparently death defying mining locations - at least in Wales anyway.
Also had a quick look at the geological map, and it seems to be associated with a basic intrusion. My colleague at work, who's a climber and a plant-nut (apparantly basic rocks=interesting Snowdonia plants) - maybe I could cajole him into being a guide.
Robin