Oh yeah so if I buy Dodonpachi Res for my NTSC-U 360 and make a Euro account and buy Black Label from the market I can switch back to my US account and play it just fine or I'll always have to log into my secondary Euro account to play the DLC?
As long as "your" console is the first one that you download the DLC to, any user using that console has access to the DLC without a connection to Xbox Live (XBLIG content always needs a connection to Xbox Live). However only the European account can use the DLC on any additional
console, be it your own or somebody else's. The issue is that you will only be able to transfer the license from one console to another as long as the licence transfer service
is operational. Currently Microsoft plans to end the Xbox 360 support lifecycle sometime in 2016 - which could essentially freeze DLC to the console that it is licensed to.
If you have no interest in the Japanese marketplace you could entertain the possibility of entering the grey area of acquiring a 2nd domestic Xbox 360 for off-line
use - i.e. have it modified to access discs from any region and use NTSC-J (and PAL) discs on it. In essence you would learn now how to manage and use an Xbox 360 in the post-support era.
Try to get an "S" model. It's probably the most reliable and has the most AV options - the current "E" model has fewer USB ports and no longer has the multi-AV connector that makes VGA, component, SCART, and S-Video connections possible.
Some other Xbox 360 NTSC-J only titles:Otomedius GShikigami No Shiro IIIRadirgy Noa MassiveStrike Witches Shirogane no TsubasaSenko No Ronde Duo
(Sequel to Senko No Ronde released in North America as WarTech)
Of course all of these are mentioned in the opening post
of this thread.
Due to Xbox Backward Compatibility Psyvariar 2: Extend Edition
can be played on an NTSC-J 360.
Any particular reason Raiden IV wasn't on your list? You may already have Raiden IV Overkill for the PS3 - but it still is just DLC (enjoy it while it lasts). The older Xbox 360 version is available on disc as an NTSC-U and NTSC-J release.