It is true, that a handful of countries can extend their visitor visa's by an extra 90 days (total of 6 months). The countries I posted, which are a few posts up,are:
- Ireland
- Austria
- Germany
- Switzerland
- Liechtenstein
- United Kingdom
- Mexico

When I read information regarding the 90 day extension in the Japanese Embassy in my own country (Ireland) it stated:

" On arrival at immigration in Japan, such passports are stamped for a stay of 90 days. This period of stay may be extended for a further 90 days upon application to an immigration office once inside Japan, but approval is at their discretion. "

I would say the chances of approval on the extension would depend on showing proof of adequate funds for the extended stay and maybe also the purpose of the extension (e.g Plan of Travel itinerary, proof of short term study etc)