The rule about joining before is easily manipulated, see gentle and decap for example. I already know you can part take on alts I have in the past as well. Part of my suggestion was imployig multiple alts in clans, so you will always be on call to save their war, sorta a guardian angel member who is contracted for tc. I don't think Decap are the only cla to have done this historically
The payment part is where I see fog, may go on for a while but if it gets to popular or you piss the wrong user groups off it will quickly be regulated and have new rules most likely implimented to stop the payment side of such a gambit.
As it stands tho, you can go ahead and do it.