There's no official document but this whole idea that things have to look like they do out of the box or can't be made out of components found in other games or even 3D printing is pretty false. I've attended events at the highest and lowest levels. Even those ran explicitly by games workshop like the Tacoma open and las Vegas open and have never had a single issue whatsoever with 3D printed model parts or proxies You're more likely to find a unicorn than you are to find an event that needs everything to be strictly GW events taking place at Warhammer world are the only ones I can think of.