The bottom line is any rule against any of this is unenforceable anyway. People can do it now without there ever being knowledge of it. As Mike says, as long as the original and future trades stand up to scrutiny on their own, it can happen. You can't police it. Obviously whoever engages in it is risking a veto on the second unannounced deal, but if a private world wants to do it, nothing Admin does or tries to do will help.
As far as PTBNL, that's an interesting concept, since it's done in RL quite a bit. Sometimes it deals with a player who may be currently on the DL or a is selection from a list of players, where a GM says, if I make deal X, I'll take player 1 and if I make deal Y, I'll take player 2.