Alright, this thread is a repository for every theory you can come up with as to what Hoid was doing at the Yomen dinner.
My own theory:
Since we know that Mi'chelle is a Nalthis Worldhopper, (and Joshin may be a worldhopper to) and that this took place many centuries before Stormlight Archive, I conclude that Hoid was still part of/working with the 17th Shard at this point in time, and had merely come to celebrate a good friend's wedding.
Of course, since he was in the neighborhood, he might have run a few extra errands (Such as hiding a note in Marasi's underwear drawer with directions to the Pits of Hathsin, nabbing a pet mistwraith named Steve, and spying on the Set's breeding programs to pass the info on to Marsh).