While what you're saying does make sense, and I think supplementation is important, I don't think it replaces the proper meal and the proper intake. It would be giving them the wrong idea, that it may be okay to eat the junk food and then simply take the supplements.
The same thing happens in the medical field. For example, when I give an individual anti-obesity medication--there are a couple of pills out there--what tends to happen is they go back to eating their own junk food, they start eating their pastries and cake, because now I've given them the magic cure. I fear this may occur if what we do and how we educate the lower socio-economic groups is telling them to eat what they want, eat the junk, eat the empty calories, and then take the vitamins.