From LanguageHat, and from the Wikipedia page therein mentioned (http://en.wikipedia.org/wiki/John_Doe#Informal_names_for_unknown_or_unspecified_persons_in_various_countries.2Fregions), I found out how Japanese people refer to someone like a "John Doe."
山田太郎 (Yamada Taro, a common male name),Neat. I like 名無しの権兵衛. It sounds like it's out of a fairy-tale.
山田花子 (Yamada Hanako, a common female name),
名無しの権兵衛 (Nanashi-no-Gombee, 名無しの means nameless, and 権兵衛 is a rather old-fashioned male name.),
何野某 (Nanno Nanigashi, old-fashioned)