I suggested this concept! It was in isotropic chat, which might be why that didn't make it into the secret history (there's no hard record to look back through). Anyway, yeah! Obelisk!
As the secret history states, originally it was "Kingdom card" rather than "Action card". My suggested method for randomizing it then was just: whichever randomizer you drew next. From the Adventures rulebook:
Event cards can also be shuffled into the randomizer deck. Events are not part of the 10 Kingdom cards used in a game; when an Event is turned over, put it on the table but keep turning over cards until you get 10 Kingdom cards. For normal play we recommend using at most two Events per game; skip any further Events.
So if you drew Obelisk, it meant that you were still going to need at least one more Kingdom card, and the next one you drew after Obelisk would be its target. In practice I'm not sure anybody actually shuffles Events/Landmarks in with their other randomizers (I sure don't), but it seemed clever at the time. I just use a d10 now, re-rolling when I hit a non-Action card.
Anyway, I liked "Kingdom card". Donald and I had one game where Farmlands were worth 4 VP. Good times. But most players don't know what "Kingdom card pile" means, so what can you do.