Apparently here in Korea there is a similar common rule to in Japan forbidding people with tattoos from using public bath houses.
I was telling a couple of traveller friends in my hostel here about Korean jjimjilbangs and they really want to visit one, but they both have tattoos.
Korea is still very conservative but individuality is gaining momentum. There are places doing tattooing and piercing in the student/nightclub zone not far from where I'm staying but "normal people" definitely don't have tattoos yet, even much less so than in Japan, where western cultural trends are picked up a few years before Korea.
I've heard in Japan that the reason is because traditionally only gangsters have tattoos and a solution is to find the places owned by yakuza. But Korea is not Japan and I don't think there's an equivalent to yakuza here. (It turns out there is, "Kkangpae".)
If some saunas accept tattooed patrons in Korea, how would one locate them? At least in cosmopolitan Seoul it should be possible.