Afaik, BD region isn't the same as the DVD region logic, which DVD could have R0 (aka Region-Free, or All Regions) discs. Those "region-free" BDs were actually labeled as all A, B and C region at the same time, that's why any BD player can play them. And since BD license does not allow manufacturers to build players without region protections and Microsoft is a copyright law abiding corp, Xbox should always be manufactured in either region A, B or C. I doubt you can modify the region w/o a hardmod. (Which isn't worth that much as Xbox One isn't technically a good BD player.)
And dev mode is just an official way for sandboxed userland homebrew, the privilege won't be high enough if it can be done via software.
You can simply buy a region A player, which is quite cheap these days.
