Well you can put whatever you like in the envelope, but it doesn't mean the charities will get anywhere close to the full value. Roughly speaking they are only likely get close to full value for banknotes of USD, Euros, BOE polymer, and the home currency/ies of the airline involved

I am not privy to the details of how BA or Qantas handle their charity collections, but I do know that a large proportion of the collection from another major airline ends up on ebay... somehow loads of collectable stuff gets donated and then it is sold on for multiples of face value, with the profits of that going to the exchange company, while the airline's charity arm gets around 50% of the face value