Hmm...I've never heard of that. I guess it depends on what you consider to be "live" rock. You won't have any of the invertebrate growth on rock in FW
...no coraline ect... However, if you add porous rock to a FW
tank, bacteria would colonize it quickly and you might even realize some nitrate reduction in the inner regions of the rock. The problem with this is that most of the porous rock is aragonite or something similar and will cause pH problems with most FW
tanks. African cichlids would be an exception to this as they thrive at a higher pH. Another potential problem would be algae growth...it would be very difficult to remove from this type of rock without resorting to drastic measures.