Ok, now this is weird. I just had one of my sims without Legendary Host throw a bachelor party. I didn't promise him the wish in time, but when the party was over (it was long, like from 5PM to 4AM), I got the message and moodlet from before UL, so I get the feeling it would have triggered.
Now I'm wondering if there's another requirement, like number of guests. The other two parties I tried throwing were around four hours long, but they also only had two people invited to them. The bachelor party had a full ten.
The strange thing is that some of the people at the party actually didn't have fun at it, so it wasn't a perfect score like the other two parties were. I don't get it