I've used 1/2 inch dia. polyethelene rods (like they melt to fill scratches in the _base_ of skis) slipped inside the webbing to form a step. This material is stiff with a bit of flex and is resistant to cold, so it won't break. We've also used 7/16 or 1/2" rope through 1" webbing to form steps.

Another option that works quite nicely is to slip the webbing through a short section of rubber garden hose. The pink or black RUBBER hose is best, not that green plastic garbage they try to sell you. The hose gives the step stiffness and makes it a bit more slip resistant. This setup isn't nearly as pleasing to look at as Nick's though. Nice workmanship on your project, Nick.