I think that every left behind parent should put a picture of their child(ren) on a T-shirt written in English and Japanese "give back my child(ren)", then walk around in the city where their ex's and ex's family are to know to reside. Or course, go with some friends to prevent arrest for "creating a disturbance". It would also be a good idea to invite the news papers.
FRIJ recommends you also
visit crn japan,
who are fighting international abduction to Japan and working to assure children
in Japan of meaningful contact with both parents regardless of marital status