Event editing: Invisible NPC like in Narshe?

Ah, that's a good tip, I'll try it out immediately. Only problem is, the cave is reachable right after obtaining the airship, so that NPC's memory bit would be needed to be set in the 'get the Falcon' event as the latest possible point in the story, right?

Yes that is correct.

Damn, my only test save right now was before defeating the last dragon. Oh well, guess I'll have to replay most of the WoR again in order to properly test it. I'll be using the NPCdata.txt document to check which NPC bits are unused (if a bit is not found in there, is it save to assume that it's really unused? Same question for the ChestData.txt?).

You could always just set the bit temporary in a current event like a NPC dialogue for testing purpose, it would be faster than replaying most of the WoR. Then after if you know it work, it's 99.9% safe to set the bit in the falcon event without another testing. Treasure chests and NPC bits both have a range near the end of what FF6LE allows that is unused, what I used to do is select the last bit in the editor and go downward for other bits. You method of double checking with Imzogelmo's docs is also good.

Too bad that map space is so limited, since the rom expansion gave me really a lot of free space that would be more than sufficient. Is there really no way to tell FF6LE to use this free space at the end of the rom for maps?

You could use FF6LE Rogue CE which allows various expansions including tilemaps.

Edit: For the location name, I thought setting the message on the map like you did was enough to trigger it in-game.