https://www.youtube.com/watch?v=Qfcet5hf5bs&list=PLACD7BBE7BB81BDC1&index=1&feature=plcp may be what you are looking for. Check about 13 minutes in.
The video wont load for me. :/
Unfortunately, there is no "IsChair" keyword. You will probably have to add a keyword, or just set up the quest alias as a specific reference to the chair you want the player to sit in.
One way to handle it is to have all the other actors in the scene do a "SitTarget" on the chairs other than the one you want the player to sit in first, then have one of them ask the player to join them, and leave only one available seat.
I have a specific alias pointing towards it. The issue isn't telling the player which chair to sit in, it's knowing when the player is sitting in it. Then I can disable their controls and start the scene.