The Cafe Orleans is definitely open; I asked for the latest reservation available on Halloween night and they gave me 10 PM!

I moved it to 5:30 for now (6 PM party start day) because in 2010, the park just seemed to get busier after the party officially started. (I'd expected crowds would vanish, but the park seemed very busy when the party started, so I don't mind missing a little bit of party time while we're eating.)
If it turns out party crowds are reported as lower than I anticipate, I may change that reservation to 5 or even switch to another day during our trip and do counter service for the party.
I believe French Market and Rancho del Zocalo are CLOSED due to party happenings.
In 2010 the Bengal Barbecue was open. I'd have to guess
most counter-service places are open (aside from the two I just mentioned as closed), but I don't know about sit-down.
Have a great trip!