Rules update:
I've fished around for the official rules regarding modified cards and this is what I could find...

UTR 28 states that:

"Cards used in a tournament may not have writing on their faces other than signatures or artistic modifications. Modifications may not obscure the artwork so as to make the card unrecognizable. If modifications to a card are deemed by the Head Judge to constitute outside notes or unsporting conduct, the player using such cards will be subject to the appropriate provisions of the DCI Penalty Guidelines."

UTR 32 states that:

"The Head Judge is the final authority regarding card interpretations. See the DCI Floor Rules for the appropriate game for more detailed rules regarding how cards should be interpreted. If the Head Judge determines that a player is using non-English–language cards and/or misprints to create an advantage by using misleading text or artwork, that player will be subject to the appropriate provisions of the DCI Penalty Guidelines."

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