Well, not only do I believe that that won't fly (if we're considering the likelihood of the piece actually being added to the game, and if we're not considering that then all piece-making rules are completely arbitrary), but even if we ignore that, there is still an issue:
Black's Assassin (or any unit that uses the scan order to determine things (which is also hidden information, which should not occur)) will function differently than White's, because the scan order puts trigger priority for Black on a space behind it while White's would be on a space in front of it.