I think the best way to do it is to create a new mechanic that does what those cards are trying to do.
I present: Hidden.
A card with Hidden enters the battlefield hidden. While you cast it, you hide it from your opponent, and you place this reminder token on top of it.
It's important to remember that a hidden creature does NOT count as a face-down creature, even if it is turned face down so the opponent can't see it. Face-down creatures have very specific rules that affect them, and they don't affect hidden creatures.
Here are some examples of cards that would have Hidden:
See if you guys can come up with some other cool things that can use Hidden well.
*Post edited to fix wording and add clarification.