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

and : 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 one would search for 龍が如く5 . 

Be warned, major spoilers!