In my humble experience they could teach a fifties school master a thing or to. An example:
I had interlined on to BA at SIN from SQ. my BA sector BP was printed on SQ stock (travelling in J). No problem at SIN, getting on the plane whatever. The warden at the lounge took one look and went 'not valid here'. I tried to plead my case that I had come from a BA flight. The answer - you CAN'T have done, you'd have a BA BP. After a couple of minutes haggling she agreed to check the flight details......and then let me in!
The lesson? The stock answer to anything outside the ordinary is 'no'. But good luck trying to twist their arms.