I've wondered pretty much since the beginning whether Arakawa is familiar with the Anglo-American military usage of "mustang" to describe an officer who started out as regular enlisted and climbed the NCO ranks before accepting a commission. What with Roy's oft stated ambition to gain ever-higher rank, that made a lot of sense.
Then ep thirteen came around and I started wondering just how far up Roy's ambition extends. If, as I personally suspect, he's planning an organization-wide shake-up of the military and the State Alchemists, possibly even abolishing the institution of State Alchemists, then either he thinks the King will go along with this or else someone else has to become King.
Now that would be going mustang with a vengance.