As far as the magnet/impeller concern goes, I've got an old powerhead I could test it on, and if it screws it up I haven't lost anything.

I'm thinking MagFloat for the outside, and perhaps a metal bracket on the inside? I just have to come up with a way to cover it. Could I "paint" a thin layer of aquarium sealant on it? I've never worked with the sealant before, so I don't know if that would work of not. Any thoughts?