But it should usually be possible to move the magnet toward the hub (perhaps toward the rim) and find a spot where it can be close enough to the sensor, but there could be unusual cases where it can't be done, I suppose.

I had this problem with my cadence sensor. I zip-tied a strip of wood (maybe 3/8" thick) to the chain stay to shim the sensor out so it was close enough to the magnet on the crank. I used a little piece of cedar, which is easy to work with, and doesn't mind getting wet (and it's light, if that matters). I carved the back side so it was concave, to conform to the shape of the chain stay, and carved the front to match the concave back of the sensor. A strip of inner tube between the wood and the chain stay keeps it all from slipping around. Actually, if I hadn't been in a rush, I'd have soaked the cedar in warm water for a while, and it would have just bent to shape when I cinched down the zip ties (this is why wooden canoes are made of cedar: it's easy to bend). Not everyone has scraps of cedar lying around though, admittedly. Some kinds of wood wouldn't hold up well enough, probably. Cedar, maple, cherry, and ash would be good. Pine would be bad, probably.

But HillRider's suggestion to use inner tube (perhaps several thicknesses, if needed) seems like a good one, as well--much simpler than fiddling with bits of wood, I guess. Why didn't I do it that way? Maybe because I felt like fiddling with bits of wood.