I think you just make several events and have them all block one another.What I'd like to do is randomize it slightly so you aren't see the same quote over and over for a set condition. My thoughts are to have several single-choice events for the same condition (e.g. on a river) and let a random one fire to set the tag. I know it can be done because the old Events and Decisions mod had the randomized settlement events but, looking through that mod, I cannot figure out how it was set up. Any hints?
I don't remember if it works like this but it might be worth a test: if you give the event all empty text strings, does it just not generate any popups at all? I.e. a silent event (usually this is done with LUA but I wonder if it can be accomplished in xml or if this was overlooked)