I took a few more pictures of both the old and new sensors, still poor but hopefully a bit clearer. Here is the old setup:

And this is the new hall effect sensor arrangement, already assembled on the pedals:

The scratched disk under the flux concentrators is made out of two layers from a CD jewel case that I glued together and milled on a mini-drill to the shape of the washers, and acts as a spacer. I scratched the surface to give more bite surface for the CA glue to hold the flux concentrators in place.

The hall sensor used is the Allegro A1302, not the most sensitive one but apparently it's good enough. I have a rougher (essentially the magnets are attached to a short shaped shaft through the C shaped piece of tin, while the hall sensor is attached to a plastic bracket holding it between the magnets) equivalent of the old arrangement in the throttle, pitch and roll axes of my Thrustmaster HOTAS Cougar and I get fine control through the whole range. Probably the circuitry of the Cougar and its automatic calibration play a large part in this.
While I was at it, I have added a few more pictures of my rudder pedals, with the new footrests salvaged from a pair of CH (non PRO) pedals, which gives me toe brakes, and the new "soft" centering system. I have also added some end stops. The shape of the levers already ensures that the pedals can't exceed a maximum travel, but this gives more positive and repeatable rudder limits