Apologies Qunli if my post offended you in any way. While I do agree it's good to remain in positive thinking, but I also think it's good to offer 'realistic' advice. From what you say your friend who visits may find it easier to camp and so on because he has friends he is familiar with, possibly may also know some of the language.

For others (like myself) who have little or no knowledge of the language & with no connections to Japan. Plans of travelling by foot and also tenting may only come across as a problem down the line (e.g what if authorities get involved?) I know the Japanese people are well mannered and friendly, but I really don't think alot of finding the idea of a stranger making camp on their property would be seen in a normal manner. If foreigners have a hard time trying to get a apartment in Japan due to language barriers, how is one supposed to enquire (out of the blue) about using a tent on someones land?

I just wanted to offer the OP alternatives to what he may be able to do. Even before I went to Japan, myself and my friends had thought about doing what the OP wanted to do but it didn't work out because 1) we couldn't find any information about it and 2) Since we were going to a foreign land with little or no knowledge of the language we knew it was just going to be unrealistic.

We did the next best thing (which I suggest to the OP) and that was to buy J-rail passes and interail around the country. We got to experience alot of the country in a short space of time, we stayed in hostel accomodation but also since we had a few days in each place we got to explore, hike and experience the surrounding areas.

The above could be done on a visitor/tourist permit, but since you cant work on one you would need a big sum of cash with you. It could also be done on a Working Holiday Visa but you need record proof to Japanese government that you have enough money to support yourself. The good thing about the working holiday visa is that the length of the visa usually lasts a year and you also get the chance to both travel and work.

Another option the OP can explore are charity run expeditions of tours/hikes throughout countries. I remember seeing one for Japan also but it was quite a long time ago and can't for the life of me remember the name. But it could be found by trying Google though.