I don't think we're entirely sure exactly what Spirit of the Ancient Golem looks like and is made of in the physical world! That is up to you to depict, and you're welcome to use your artistic freedom with that
Yes, you're free to depict champions wearing modified costumes, and even completely new themed skins if you wish. We have no requirements on that.
Sorry for the response time--I couldn't log into the forums over the weekend! I'll always be faster to reach via twitter.
This is my tweet:
@RiotMsPudding Is Spirit of the Ancient Golem a blue fist with a blue metal sheet and a sapphire or the metal sheet and jewel? #DigiArtFAQ