You say you don't want to railroad them but then ask if it is unethical to cheat the rules of the game so you can put them on your railroad. That makes no sense.
And yes it is unethical. Imagine a similar scenario, you are a street samurai, no matter how good you roll, my NPC beats you. I don't roll dice, I just say he beats you. Would that upset you? (Hint: Yes, it would.) Also, at that point, why bother playing a game, just write a book.
The truth is, you're not going to be able to hide anything from your decker if he rolls even moderately well, if he looks. Don't depend on it. Have your story unfold some other way and stop being railroady.