>>12487890They certainly could have, it's a matter of will, really. You have to think, the only Pokemon allowed to be traded have to be non-gen5 with no gen 5 moves.
On top of that, they would probably have to restrict based on any extra ribbons or extra things like that added in gen 5. I can't remember now, but is there more info on the Pokemon's summary page in gen 5 than there is in gen 4? Like where you caught it or something? Because if that was added, it would be lost when sending back to gen 4.
