For anyone interested—I ended up going with an Arkel Randonneur rear rack and a basket with a MIK quick release plate. I picked the Randonneur because it gave increase flexibility on the angle of the rack, but I think a Topeak Beamrack (or similar) would also work well.

I can't post pictures until I've made 10 posts, but the bike folds the down to the original size by rotating the seat forward and removing the basket. If you keep the basket installed, the bike can be used in a sort of "shopping cart mode", similar to a folded brompton with basket (not as wide of a stance, so won't be as stable, but seems manageable).