Wellll.... I don't know for 100% certain but I'm going to assume that:
Ever see those activated charcoal filters? For bongs? It cleans the smoke and smoothens out the hit? If charcoal did in fact have THC absorbing properties like that then I doubt ROOR/other brands would make charcoal accessories.
But no, I think your shit out of luck if you think youll piss clean if you smoke a J and eat some powder..