I was looking for exactly the same thing in advance of a multiday trip last year, and found the pickin's for foldable 28s to be surprisingly thin. In addition to others mentioned here, have a look at the Panaracer RIBMO, which comes in a variety of flavors including a foldable aramid bead 28mm. It's a little clunky compared to the Gatorskins (which are great), but makes a very servicable and cost-effective backup for the just-in-case.