From the Sands of Dorne preview
At Prince Doran's Behest reads:
When Revealed: Choose and reveal a new plot card. (Cannot be cancelled.)
The RRG section on Plot Cards reads:
When a player reveals the final card of his or her plot deck, after all just revealed plot cards have entered play and all necessary “When Revealed” abilities have resolved, each plot card in that player's used pile is returned to his or her plot deck. The just revealed plot card is not yet in the used pile, and remains revealed.
If you have to choose and reveal a new plot card, but your plot deck is empty, will you be able to choose a plot from your used plots, or will the when revealed ability fizzle, leaving you with 0 gold, initiative, claim and reserve?