Yes, I’ve tried the smaller design just inflated. It was Alpacka’s offering that made me confident an inflated footrest would work. I filled the bag as full as possible without using a mechanical pump. The issue is that your feet don’t apply even pressure on the footrest when paddling. It only felt solid once you added an object and pushed out all the air.
Maybe it’s possible to get it firmer. There may also be a solution by dividing the internal area into two separate compartments. You could probably use an internal wall to limit compression to a single direction. For me, retaining foot space storage is a must, so these experiments were never tried.