B12 is like a homeopathic medicine-you only need a little bit every day. Its due to our modern refined food that we go many days without it, then need to catch-up. There is a chart somewhere that shows how certain vitamins help absorb other vitamins, likewise there is a chart that shows minerals. It all starts from cobalt, which is found in B12.
The intestinal tract may work, but we've exposed ourselves to many environmental toxins over the hundred years since B12 reduced drastically I am not sure you could ever cleanse yourself enough to host it.
Korea And Japan are doing research along these lines right now for probiotic " Antibiotics ". An extract of KimChi kills the Avian Flu.
Japan is using extracts from compost as household disinfectant like vinegar.
In the days of yore a peasant remedy to certain disease was to actually 'take of the earth'. Ayurvedics still prescribe it at times.