Fail 2FTS at the right time and you can go to the army!
That has happened in the past, however Army has recently seen the light and reversed the policy of accepting another services rejects.
what would be the chances, hypothetically speaking of course , of someone transferring from say RAAF/ Navy to Army during BFTS or at the completion of?
Mate, the answer is always "no" if you don't ask! BFTS's establishment includes a senior Army officer (Major type). Go and knock on his door.