When the invading infection, ie. flu virus, is responsible for the toxic load in the system then it makes perfect sense to attack and eliminate the virus with a non-toxic substance, ie vitamin B12.
What you seem to be suggesting is to just let every cold/flu take its course and don't interfere. What with the appearance of various potentially life-threatening strains of flu virus this seems to me very bad advice. There's a lot of research suggesting that high doses of vitamin D3 can ward off influenza and elderberry extract can act as a natural antiviral. Would you also include both these as "any foreign substance applied as a treatment"?
I'm still keen to hear of anyone's experience with or thoughts on nasal application of B12.