Can't be done for restaurants owned by Disney or any restaurant located inside a Disney theme park. All reservations for them must be made through Disney. Le Cellier is Disney-operated and located in a theme park, so you can only make reservations there through the Disney system.
Some of the third-party restaurants (Swan/Dolphin restaurants - you may be able to book reservations through Swan/Dolphin, Portobello, T-Rex/Rainforest Cafe, Big River Grill - for Raglan Road or Paradiso 37 you can make reservations through www.opentable.com as well as through Disney) accept reservations directly.
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