1. Horned Viking helmets are not an American invention. It's a product of 19th century European Romanticism's obsession with ethnic origins and an idealized version of folk culture. The modern misconception of horned Viking helmets can be traced to various Scandinavian and German artistic misrepresentations of archaeological findings and Romantic blendings of Classical and Norse imagery.
2. Non-ceremonial horned helmets in general are not necessarily ahistorical. The Winged Great Helmet is obviously based on a popular conception of Teutonic Knights. Examples of such can be seen in contemporary sources like the Codex Manesse and some surviving helms. Of course, as an artistic work, its reliability in communicating military reality might be suspect and the surviving helm could be a tourney piece. Alternatively, such helms could have been the preserve of nobles and would be useful by identifying them as such so that they would be ransomed instead of killed and/or by making them easily identifiable to their escorts/soldiers they are leading. Keep in mind that these ornaments are usually made of wood and leather and are not part of the helmet itself (unlike the Winged Great Helmet model). And if surviving helms like this one
http://upload.wikimedia.org/wikipedia/commons/6/6b/KHM_Wien_B_74_-_Great_helm_of_Albert_von_Prankh%2C_14th_century%2C_front.jpg are indeed jousting pieces, I don't see why similar items should be excluded from cRPG, seeing as we already have other items explicitly meant for tournaments/jousting.
