I'd be curious how those perform. As for legality, I'd say it's a gray area, but technically it would be legal as the rules are worded-- a magnetic would stick thus they're ferrous.

I would also be curious what a set of regular 2-pc rotors with a lightweight disc weigh in your size-- the 12" rotors I have from Coleman are under 12lb each. You may be splitting hairs at that point on weight & performance..