put in BA in the override box but keep getting royal air Moroc instead...
That's because BA it's NOT the airline code for British Airways. It's only the name of the *folder* which contains the *textures*, so this might have mislead you into thinking it might be the airline code, but it's not.
The airline code is the 3 letter ICO one which is used in the AFCAD, so British Airways will be BAW, see a line coming from one of the default GSX vehicles:
[fltsim.40]
title=FSDT_Staircase_BA
model=01
texture=BA
couatl.icaoprefixes=EG EGLL EGKK
couatl.atc_parking_codes=BAW,BRT,XMS,SHT,BIS
While the textures are in the BA folder, the ATC codes are BAW,BRT,XMS,SHT,BIS, which means GSX will let the BA repaint appear if it finds one of these codes in the scenery AFCAD OR if you set the airline code explicitly in the parking customization page.
I tried it now, at the default GMAD airport, set BAW for the *whole* airport (assuming this is what you want to do), which means selecting the root node on the left named "Airport", typing BAW and then APPLY and, sure enough, I got British Airways at GMAD, without even requiring a Couatl restart.