Did you ever settle on a mechanic? I am about to start a very similar campaign.
I was thinking that any magic items could be enchanted as they are normally, and just be considered to be driven by technology. Any time a spell is cast near a tech character or technology, it would force a concentration check of 20+ the character/item level, whichever is higher. If they fail by ten or less, the spell simply fails. If they fail by more than ten, they take damage for it. Maybe a d6 for every five they fail it by?
What did you come up with?
I'm also totally willing to swap concepts if you have any.
