One workaround for custom heroes in Matched Play and Tournaments is to have community guidelines that restrict certain combos or builds that prove to be abusive. Perhaps have a banned list like
No Lumineth Heroes that give units +1 attack
No Kharadron Heroes can be wizards
I'd probably go with a crossed hammer and tongs to represent their being master craftsman.
This is my alternative Zaitrec paint scheme. I didn't think the official one really suited their background so I did this one to really emphasize the "Moon Magic" aspect to them. Do in all contrast plus Leadbealcher and Retributer Gold.