[Nyororin]

They may want you to take it out at some point - or they may not really care at all. I would try to get the smallest and most plain ring you can manage and be really honest about it from the start. Hiding it is sort of like saying you know you`re doing something wrong...
Piercings are not that rare, and are not as "bad" as tattoos in public opinion. And in the end, you`re not Japanese and will not be held to the same "standard" anyway... Even by the most conservative and traditional people out there. You`re coming from a different culture with different opinions and values - and everyone knows that.

I say just be honest about it, have a very small and plain hoop, and then follow your host family`s lead. Chances are, they`ll just accept it as part of the package that is you and not say another thing about it.