I have done a quick search on the BYAP and have noticed mostly just forums on using Activated Carbon/Charcoal as a bio filter. 
I'm just starting on my journey through AP. I did a search on Activated carbon as i know of a way to make it myself. I know it works well as a filter and that it as a point of adsorption then as i understand it becomes "inactive" or another words the pores are completely saturated. At saturation point it will begin to release the stored/filtered properties. 
But the Question I have is if you keep your system in balance introducing the atved cbn during the start of the system as your grow medium allowing the bacteria to saturated the atved cbn all with proper planning and timing of introductions of fish and plants into the system if the activated carbon turned grow medium could then become a life support of sorts for plants when something does go wrong such as the pump going out. I think as an added bonus the pores being extra surface area to grow bacteria on. 
Of course I feel I must state the system I imagine does still have the bio-filter as the activated carbon as the grow medium isn't so much filter as it is exponential surface area and back-up life line.