Many of the truck dock bumpers (from my observation) are made from sections of recycled tire casings. The nylon/rayon cord is usually visible in the edge-on view. They're incredibly durable, provide some cushioning when the trailer is backed in, but they're not really compressible in the normal sense. I've seen other types made from relatively stiff rubber that have a cross section sort of like a "D", where the geometry of the shape allows it to compress, not the inherent compression of the material. Neither one of those seems ideal to your tie down application. What's the background for your bias against a steel spring?
Maybe you could look to the bicycle world? There are plenty of suspension forks that are based on compression springs made from elastomeric polymers. The "springs" are made in the form of little pucks with a hole in the center for the shaft. They're available in various Durometer grades and can be stacked. The whole idea of those little disks is to provide controlled energy absorption in compression, especially at relatively high rates. They may need to be constrained in a cylinder to work well (otherwise they might be so soft they squish out sideways too much -- dunno the details here, but probably easy enough to check).
