As I understand it the crystals in nappies are actually a silica powder, (as found in the little packets you get in with new electrical goods/trainers etc) in which case it probably wouldn't be accessible to the plant as it would take a fair amount of heat to re-release the water - it becomes gel-like when wet. It might work if the plant roots were in direct contact, but I can't see it being much use placed under a pot.