Eh, I'd stay away from events that are too rare. Leads to save scrumming and just frustration when you need a specific event for the game to develop the way you want.
The game really supports this already. You can use the item editor to create whatever you want, assign it what rarity you want, and what cost you want. No reason you can't create a consumable item 'A day with the googly-eyed hypnotist' mark it as very rare and assign some incredibly high cost. Assign it a very high obedience score, maybe have it assign a controlled trait or so, and go to town. Check the item shop daily and you'll get your rare event.
I'm sure it could be done with a script, just not sure if it's worth the development time to create a new feature that duplicates something already in the game.