List of unused captions

Some more that are definitely not used:

36 (apparently dummied dialogue from Terra in the flashback when Kefka enslaves her)
38 (apparently dummied dialogue from Cid chatting about Magitek in the flashback
2345 (apparently dummied dialogue from Sabin to "Gerard" in Tzen)

299 *is* used - if you pick up the piece of paper in the Returner hideout.

Finally, 2500-2522 are used, technically - they're the various combinations to carving stuff on the stone in Darill's Tomb. However, one could presumably use a single generic response like "it's just gibberish" and use an event editor to use that dialogue for all the wrong combinations, and that would free up the others for use.