I'm not too sure that just talking with and having a penpal in Japan will automatically guarantee you a rent free place to crash while visiting there.
I dunno but I think it's a lot to ask of someone who isn't a best friend or relative especially if you will be staying with them for a couple of days or perhaps a week. Not many will agree to something like that especially if it's someone you've only met online (not to mention dangerous).
I think it might make more sense to try and stay at a "gaijin house", "guest house" or other similar housing arrangement. They aren't all that bad and aren't like the "capsule hotels" or other "hostel" type places you may have read about. They also do not require a Japanese guarantor or local contact.
Might be worth considering rather than crashing some stranger's place.
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http://www.freesianet.co.jp/gaijin/
Guest house Tokyo - Gaijin house