I absolutely love the new Arachnarok Spider model released for the Fantasy Orcs and Goblins codex, and think it is filled with great modeling potential. While I have never completely embraced the counts-as phenomena, I was struck with an idea of how I could use such a great model in my Codex: Chaos Daemons army in 40k, but I'm unsure about the rules. Let me state that I normally only play in casual games, and know the protocol for getting counts-as units accepted in a tournament style game, but when the model it is representing has no official model or base size listed, there is a nice gray area to work with. While I originally wanted to use the model as a counts-as Great Unclean one, it was pointed out to me that the generic "Greater Daemon of Nurgle" is the "accepted" model and base size. My question is thus:
The unit in the codex Ku'gath the Plaguefather has no official model (outside of, I guess, that giant resin model from forge-world) and maybe the Arachnarok could fill the role with some nice creative modeling, green-stuff, painting, etc. Provided the majority of the model could fit on a 40k base such as a Stormraven/Valkyrie/Trygon base, would it be an acceptable counts-as model as far as model and base size? I think it fits the fluff in a very cool and creative way. Just don't glue on any goblins, green-stuff on some boils & nurgle symbols, add Nurglings to the base and voila. You now have a giant daemon spider running around dripping poison on its enemies and spawning little nurgling/spider things!
So in summation, is such a creative thing acceptable rules wise in the game? Thanks for reading!