What about, you play as the Orkz? Then you get around the problem of other races technology, seeing as Orkz are natural scavengers and will utilise pretty much anything it would be a safe bet to 'play' as the Orks, therefore you can use any races ships and weapons without damaging the 40K universe lore.
Though you could still do missions that are for Imperial/Tau/Necron/Eldar/Chaos/Space Marine/Draugir(Space Dwarfs)/Rogue/Kroot/Tyranids and there are some others that could be used, that are mentioned in law