It turned out that the rowingbike is a great solution for people that can only use one leg. There are over a dozen rowingbike riders that only use one leg to propel the rowingbike. Starting off is hard to learn, but the effort pays off.
Since the rider also uses arm and back muscles, speeds are not much lower than when all four limbs are used. Nowadays each footslider already has extra attachment points for additional bearings. These are necessary to cope with the high asymmetrical forces these riders exert on the foot slider.