From The Horse's Mouth (So To Speak!)
Possibly slightly off-thread, but in the early 1970's I was lucky enough to meet a friend of my parents, an RFC pilot and founder member of the RAF, Percy Alderton. He told me that he was already in the Army when he wished to remuster to the RFC - I don't know whether or not he was already commissioned at that stage. His selection process was, however, quite simple. He was sent to an indoor riding school in the basement of an MoD building in Central London (I can't recall which one) and given a fairly comprehensive riding test. He was told that, as he had a good 'seat', he should be ideal for flying duties and that was that - he was remustered. He later flew Sopwith Dolphin's and eventually ended up as a Wg Cdr Education Officer at RAF Jurby before retiring.