I browsed the Yahoo Auctions japan for bikes with wheels smaller then 160'.
I have found this interesting A-Bike Gadget:
http://page5.auctions.yahoo.co.jp/jp/auction/e93758702
And its other part always from the same guy:
http://page18.auctions.yahoo.co.jp/jp/auction/w39087527
http://page4.auctions.yahoo.co.jp/jp/auction/d99118506
You can tanslate the pages from Japanese here:
http://babelfish.yahoo.com/
To travel around in Japan, it helps a lot that the bike stands while you buy the subway ticket.