How to purchase and convert a JP kindle book

Log in to amazon.co.jp and go to Account > Manage Your Content and Devices (Digital Content) > Settings tab, set the "Country Settings" to Japan.

Find a Japanese proxy/VPN. If you're willing to pay, PIA has servers in Japan and their prices aren't bad. If you're not afraid of going the free/public route, there's vpngate.

Before clicking "purchase", turn on your VPN. Apparently that is the only time it is necessary (i.e. browsing and downloading the books without the VPN is fine). I've bought ~150 books following this rule and I haven't had my account locked yet (knock on wood). HOWEVER. I only just started using Kindle for PC and I haven't done much research yet, so I don't know whether the same rules apply to it.

I think downloading and logging into Kindle for PC should be pretty straightforward. I used my VPN when logging in and downloading books, since I don't know whether it's the same as the patched app yet.

Once the book is downloaded, an ".azw" file should show up in C:\Users\<USER>\Documents\My Kindle Content" where <USER> is your username, if you're using windows 7. I'm sure google can help if you're using a different OS.

Download and set up Calibre and the DeDRM plugin for calibre. There are instructions on how to do this on the respective sites.

Open calibre and add the azw file, DRM should automatically be removed (kindle for PC allowing JP accounts removed about ten steps here!!). You can test this by attempting to view the file in calibre, it should complain if the file is still locked up.

Convert the book to ZIP format using calibre, make sure Page Setup > Output profile is set to "Tablet", as that prevents resizing of images.

After the conversion finishes, I usually unzip the file (you'll have to fetch it from wherever you set your calibre library folder to be) and re-zip just the "images" subfolder (don't forget to copy the cover image into the folder first though).