I have heard of injecting a small amount of umm I can't recall which oil but a light laxative for constipation although if a snake is eating I'd usually guess it isn't needed myself. The thought is not that bad but I agree not necessary and the thing I'd be concerned about is how much of what. Vitamins can be toxic in the wrong amounts and some can be processed and some not. It sounds to me that the potential gain (if any) is so small and the risks are not non existent that the gain and risk weighed together leaves you no farther ahead and possible placing an animal in danger.