I really don't think the "hardness" is a problem for snakes. We perceive what we're stepping on as "hard" because there is an impact because we walk -- the footstrike. A snake is very different--the pressure of his "footprint," if we consider his body contact with the cage as such, is spread out much more evenly. Because he weighs less too, PSI on his contact surface would be way lower.
I used paper towels as a substrate for a long time and the snakes didn't seem to mind. Lots of people use newspapers too, and neither one is soft when you figure that you're only using a couple of sheets thick on the cage floor. The only reason I switched to aspen is because I've got a couple of females that pee about a gallon and I regularly found them in wading pools. When I put aspen in my cages the first thing my snakes do is pack the stuff down. They might even prefer a harder surface because it makes their form of locomotion easier.
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