tkrik - Because the amount of DM appropriate for the low-dose effect is so small, I doubt that you would get enough antihistamine to be effective. The usual dose of the dilute pediatric preparations for cough is a couple of teaspoons at 6-8 hr intervals; way more than used in the low-dose approach.
Some of the additives, especially polystyril, result in a slow release of the DM, which defeats the quick "peak" of the med required for the desired effect. I learned this directly from Dr. Hong who has done much of the research on suppression of CNS inflammation with morphinans like DM and naltrexone at very low doses.