Thought experiments in magnetic tracking.
OK, so the lack of hardware of any kind to test with is really killing me here, because it means all of this is pure speculation. However, while thinking through the issues of the hydra's tracking ...
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Unfortunately, the miniaturised 3-axis magnetometer in the Rift (and PS move, your phone, etc) are not sufficiently sensitive on their own to pick up the fields the Hydra base station puts out, and likely tuned to integrate over some time period. The Hydra controllers use a trio of orthogonal coils to pick up the three spatial components of the field with sufficient sensitivity, and amplifies this on-board before sending the amplified signals back to the base station where the actual processing is performed. Additionally, the Hydra uses timed pulses, which any alternative receiver would have to match to know which transmit coil was energised when.
If you can find a magnetometer chip that accepts external coils, you may be able to produce an unofficial Hydra controller. Unfortunately, existing devices will likely not work, or work with better spatial accuracy than a few tens of centimetres.
If you can find a magnetometer chip that accepts external coils, you may be able to produce an unofficial Hydra controller. Unfortunately, existing devices will likely not work, or work with better spatial accuracy than a few tens of centimetres.