As AOTB mentioned... no one must register. I'm going to a friends wedding this weekend actually & I know she's registered at places but don't know where because I don't live near her family or friends. This friend is like an older sister to me thus is my MOH. Still I'm not going to outright & ask her even though she's my MOH. I do know she has her own place, car, & steady job so I know she most likely doesn't need any house items. I haven't seen her house because we recently re-connected, thus am giving her $.
I forget the person who said this but agree with her. I would offer to store my gift to them at my place. No big deal. Like you said they don't have room to store their gifts there. Their family should mention through the grapevine that they're living in close quarters in Japan, and hopefully guests will get the hint. GL.