Would it work to serve ham cold with mini croissants for a large Easter open house brunch? What topping/condiments would you include?

Works for me :)

Bread and butter pickles, mango chutney, good old mayo, tomato and lettuce, runny brie cheese.

I would have butter, mustard and cheese available.

Imho, croissants are not very good for sandwiches. Mini brioches are much better. The layers of the croissant do not lend themselves to horizontal slicing.
Cold ham is perfect for sandwiches. It depends upon what you like with it for condiments. Chutneys (peach particularly) would be nice, and a full-bodied mustard - could be German style or Dijon, mostarda also goes well. And almost any kind of pickle.

I've done this and it works really well, although I pre-made the mini-croissant sandwiches. Ham, srircha/peach jam, mustard and arugula. Make a lot. They get eaten very quickly.