Determening the location of a ball contacting a surface
Not stupid idea, but use soft low remanence iron ball instead of a magnet. It's position is arbitary, nothing needs to point to some specific direction. Have an external DC magnetizing coil and some small pickup coils. Measure the pulses induced to the pickup coils. 

The hard part: You must match some complex function that gives the hitpoint coordinates from the pulse amplitude proportions. It can be a table of premeasured values and the intermediate values can be got by interpolation. Triangulation may be useful or not.

DC magnetizing might be better to replace with high enough frequency AC to get stronger signals. That would eliminate all effect of remanence and earth magnetic field. Also the eddy currents can increase the detectability. You must explore with hf magetization, which phenomena gives the strongest effect.

Iron powder+glue or ferrite ball has no eddy currents, a copper ball has nothing, but eddy currents, solid iron ball has magnetic effect that is affected by eddy currents.

Totally different approach: Learn how resistive or capacitive touch panels work

The next: have some microphones or schock sensors. Triangulate from timing differences. Actually, to get the result fast, compare with a table of prestored timing patterns and iterate or interpolate.

And still one more: One schock sensor gives the hitting time and triggers 2 perpedicular line cameras or lightport arrays. The software finds the mass centres of the shade patterns and these are your coordinates

Good luck!