While you don't get to choose the card you seek, if you only have one card with the highest mana value, you'll always hit that with seek, as Runecarved Obelisk will always try to get the card with the highest mana value left in it
Interestingly, this foil treatment has a bizarre issue some players have reported that you can rub the Phyrexian symbols off the card
He can either deal X damage to any target or cast an instant or sorcery with mana value X or less, where X is the highest mana value among Equipment attached to him
(Call LLM via self.client.chat.completions.create)
Luffy from A Fist of Divine Speed [OP-11], Miss Doublefinger (Zala) from Awakening of the New Era [OP-05], God Thread from Royal Blood [OP-10], Gum-Gum Red Roc from Kingdoms of Intrigue [OP-04], and Gravity Blade Raging Tiger from Wings of the Captain [OP-06]
It also means that if you really want access to these effects as a Commander player, you have a much more affordable route to getting a card with the same effect