Yes Keg that would constitute a valid contract , same if it was written on toilet paper & signed by same
I am sure you are an excellent pilot , solid citizen etc. But your thinking appears quite narrow in this area, that everything has to written down & certified by a whole lot of singers & dancers or it has to say "can be done" in legislation or other instruments. Otherwise you just can't do it
Off the books deals , agreements like these are made all the time in the commercial world. It's easy , no need to overcomplicate it
Various LOA are pretty Mickey mouse in many ways drafted up in five minutes, that would be one mechanism - but likely overcomplicating something that is very simple
Free world etc , if you all think it's a bridge too far or you support the hiring of external candidates