FWIW, I've never found a light that wouldn't change if I put my wheels right on a sensor line. The sensors are big metal detectors. I don't think a magnet should really affect them any more than any other piece of metal.
I know that there are sensors that aren't calibrated properly and won't detect a bike, but most will if you line up properly.
All you need is something conductive that's close to the ground as possible. That means your wheel rims. If you plant them right on top of a sensor loop, it should trigger.
http://draco.acs.uci.edu/rbfaq/FAQ/8i.2.html
They just switched to camera-based systems in the town I ride through every day. They do not detect bicycles AT ALL. In fact, I'm pretty convinced that they're aligned so badly that they barely detect cars. I've waited at reds there for several minutes with no other traffic on the road, and it didn't change until someone else drove up behind me.