That's a good question, sublunarfields! As I couldn't remember whether any of the original stories featured Freemasons, I did a quick search online. However, most of the results that came up referred to the Ritchie film, as part of it was shot at Freemason's Hall. I wasn't able to find anything referring to canon, but maybe I didn't dig deep enough. I'd be surprised if the masons weren't mentioned somewhere. I do seem to remember Holmes deducing that one of his clients was a mason, but I can't remember if that was in an original story or a pastiche.