I realized that the JP account was needed ONLY to allow access to their MarketPlace where you can initially download the demo. That is all you need on your HDD.
Afterwards, just creating another (non-JP) account on that same console, still has that JP demo show up in your Recent Games menu.
Launch! >>>> another code!
Haven't tried it in a week or so but I know certain LIVE areas were down the other morning/late night -- the day of the big Minecraft Update.
Update: Still works for me!
Yeah, i tried it again for myself and it was not working. Looked at how many "New" accounts I had on the Xbox, erased the "New" ones and tried again......worked perfectly.
Looks like you can only have a certain amount of accounts on a Xbox at a time doing this method. If anyone knows otherwise, please fill us in.