I've never liked having a cadence magnet on my crank arm because a zip strap half-way down the crank arm looks out of place there.
So, I've been thinking that some pedals (my Time's for example) have a 6mm socket on the inside of the spindle so you can install the pedal with a hex wrench instead of a open box wrench. What if I was to thread that "socket hole" so that a short cap screw fit in there with a strong (read neodynium) magnet glued into the socket on the capscrew. I guess you could also just glue a magnet right in the pedal socket too, but it might be too far away from a sensor mounted on the chain stay.
Has anyone tried this?