well, the carbon is inert (for the most part), so i don't think it would hurt to use as a substrate
after it's expended it's filtering capacity, it still has a lot of surface area on which bacteria can grow
only problem I could see is certain chemicals (several medications have warnings about removing carbon before using) can shock the carbon, causing the chemical bonds it uses to hold toxins to break down, releasing all those toxins into the water!
I think that is because activated carbon never really 'wears out', but instead it just gets 'full' under normal circumstances, but it's always trying to form chemical bonds with new materials ... so if something more reactive comes along to bond with, it will shed it's weaker bonds in favor of stronger ones with the new material, releasing junk into the water, and taking the medication out at the same time.
there was talk about reactivating carbon on the apd, but the temps needed was something like 800 °f, which is pretty darn hot