Just following up on my own email in case anyone's interested in volunteering as well.
I ended up agreeing to join 2 separate groups. One in Kitakami, Iwate called HANDS (not to be confused with All Hands) which does work in Kamaishi and one in Ishinomaki, Miyagi called "It's not just mud".
HANDS Kitakami
It's not just mud – Volunteer in Tohoku
I wanted to spread my 2 weeks of volunteer time across different groups to check out different affected areas and to experience working with both a Japanese group and foreign group. In the end, these groups worked out since they were operating in December and because they didn't require any orientation and training.
They'll even accept people who are there for only a day so give them a shout if you'll be in the area!
Of course I'll report back with a full write up when I return.