Unwritten Rules are Not Part of the Game

In all games, within game contexts, you may only do things expressly allowed by the rules. This is what it means to have rules; it is the covenant you have agreed to by agreeing to play. You can play tic-tac-toe in a van while yodeling, but putting a Z in a box is out of the question. It is not up to any rulebook to say that you can't use a memory aid; rather it is up to the rulebook to specifically allow it, or else you can't use one.

It doesn't matter how much the game for you is not about this memorization, how much the memory thing seems tangential to whatever fun the game provides; you do not get to use anything other than your brain to handle that memorization, unless of course you are explicitly playing a variant.

You also do not get to - and this is important - scrawl notes to yourself on your belly using your own blood. Games between players are played between players, and "players" do not by default include notebooks or pencils, even makeshift ones that are constructed from the players. Expecting all rulebooks to repeat this is nonsense, and anyway would offend people who don't like to talk about blood.