Kava is actually a good local anesthetic...you could rub some extract on your skin to numb it a little. Or that might be a terrible idea, since the kava would get into your system when you get pricked by the tattoo needle. Yeah, terrible idea, don't do that. Just lidocaine cream or whatever the tattoo guy has is much better... It would probably be OK to drink a little kava to relax if and only if you don't take your opiate/acetaminophen pill with it.
I looked in the literature, and found a couple vague references to kava having anti-platelet effects, but nothing convincing. Most of the hits you get on PubMed when you search for "kava antiplatelet" or "kava anticoagulant" are general reviews of herbal medicines that talk about Ginko having those effects, not kava... If someone knows of some definitive literature on kava being a "blood thinner" I would be interested to know.