I haven't played Fallout Tactics in a while, but I recall that the Midwest Brotherhood was less east coast "unambiguous good guys who fight for justice and muh freedoms" and more west coast "moral ambiguity, isolationism and technological greed", but things change over the decades.
I don't see why the NCR couldn't send one of their Vertibirds over, since they found a bunch at Navarro and have likely made factories which make similar flying vehicles in the Big Circle or Shady Sands.
My original idea was a tribal from a collective similar to the Legion, maybe some centralized gathering from Texas or the South, but if you want NCR here's my idea:
NCR Ranger as my character, sent alongside a representative to the East Coast on an information gathering and connection-forming mission, which goes predictably awry and my character is the only survivor, dead set on completing his expedition despite his utter lack of diplomatic experience.