Test report:

I got a 9 inch diameter thin sheet of Al (ex- pie dish), spun it with a
hand drill. I've got this seriously powerful magnet from an early
voice-coil HDD (much bigger than typical now), that I held up to one
side of the disc.
The magnet slows the drill somewhat - I can hear the change in pitch.
The magnet tends to want to "push off" the disc.
As proof of principle it's encouraging. It'd need a much larger
diameter, especially if I can only get smaller magnets, but multiples
would help of course (I think?).
Also I'm guessing the thickness of the sheets would need to be at least
1/8 inch, as the push-off pressure would likely warp a thinner disc.

But hey - all that aluminum is threatening the purse limit again!
I wonder - would it work if a number of pieces were attached to a sheet
of ply? Then we could make a number of "rings" from less material.
I've gone cool on the bicycle rim idea, thinking there's not enough
brake area.

Jordan