What you describe is exactly what the "without blunders" mode attempts to do, but evaluating every possible move for each player to a useful search depth takes a long time, so it doesn't search very far.
Also, the more you restrict the space of allowable moves, the more moves are required to encode a given message.
Oh I didn’t notice that option, cool.
By the way, what was your motivation for creating this?
If the moves actually made sense, this would be a lot more useful. There's probably something that can measure how good a move is, so I guess what you can do is make sure that the moves you use to form a message are above a certain threshold. That way, someone looking into the moves wouldn't immediately know that they're being used to encrypt messages. But honestly other than that, I dunno what you can use it for.