Hi!
I am very interested in what you came up with, or come up with. (Would you please email me at phil@philtroy.com if you did/do come up with something?)
One approach (similar to yours without requiring a motor) would consist of:
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Put an led at the edge of each pin
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Place a digital camera where it can see all of the leds
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Measure the brightness of each led, either with an array of photodetectors or with a digital camera
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I am guessing the brightness will decrease with distance so I would try to apply the inverse square law to determine the distance, keeping in mind that pins on the outside of the matrix would be further away even when fully extended. The accuracy of this approach would be enhanced by putting the array of photodetectors as close as possible to the leds at the end of the pins so that there would a large enough effect for even small changes in distance.
A second approach would consist of:
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Building pins using a resistive material
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Applying a power source between one end of the pin and the surface of the board
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Use a multiplexer to select the row
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Use a second multiplexer to select the column
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Measure the current and convert it to a digital signal
All the best . . .
Phil