As of writing this there are a couple people uploading playthroughs of RGG5 on niconico video.

http://www.nicovideo.jp/mylist/34909793 : commentated

and

http://www.nicovideo.jp/watch/sm19522752 : part 1 (uploader’s channel is private so future vids one would have to search for 龍が如く５　プレイ動画  followed by number.)

There are also a bunch of people livestreaming the game. On live.nicovideo.jp one would search for 龍が如く５ . 

Be warned, major spoilers!