I have stayed one night at Takimikan which so has outdoor Onsen. Since Ginza Onsen is pretty Japanese and traditional, I would assume tattoos are not allowed. In more touristy (with lots of foreign tourists) places like Kiroro Ski Resort in Hokkaido, there are notices that say that small fashion tattoos are okay. We are talking about big ski resorts like Kiroro. Places like Ginzan mostly invite Japanese tourists with very few foreign tourist. If yours is small enough, you may be able to get away with it. When I was in Takimikan, I was in the Onsen twice and saw Only one other person in the Onsen. So if no one is there at the time that you go to the Onsen, it probably would be okay. Hope this helps.