There is a product, Fog-X, by the makers of Rain-X, that prevents interior fogging, but I wonder if the product was ever meant for headlight lens application.
Just as aside, I just unboxed a pair of Bosch Euros for my W201 and didn't see any signs of "venting" on the assemblies. While I admit that the plastic housing probably does have seams or some sort of air circulation to vent the heat generated by the bulbs, vents are'nt really visible.
and, have euro light owners noticed a difference in the color of the euro orange side markers versus the US DOT ones? The euro corners seem darker orange.