Hi Phlimm. Well I can't speak to all the issues surrounding what is considered "public domain" and what isn't, but I don't think the Guild has a usage policy per se. Many contributors here have however made their work available for private use and distribution under a Creative Commons license (some of them even advertise this in their sig). On the other hand, some of the maps here have been done as commissions for commercial publication. If in doubt, best thing to do is just PM the artist whose work you would like to use and ask permission.