Guess you missed out the note about the EU/US X/Y patch being region free and that people have dumped Japanese CIA update file for other Japanese games.
To add on to ^ reply,
I'm using Pokemon X UK rom (Contrast), on US device,
using Pokemon v1.3 patch dled from US eshop.
and it works.
proof enough??
AND even if they make the patch jump through loops to block different region rom,
we can install the .cia using devmenu, cause BBB probably will dump them.