Bruce Turrentine wrote:

>  I'm considering simply running the wave through a germanium diode
> rectifier and watch the trailing edge of the first pulse.

  Think not in terms of Germanium diodes.  They are obsolete, and for
good reason (especially in the heat of an automotive environment).

> When the wave falls, the slope is nearly vertical, getting me close to
> the zero crossing.

  It'll be just as good with a silicon diode.  But what's the diode for?
Feed your reluctor through a resistor (mmm, about 33K) into the base of
a transistor, and put a silicon diode in reverse across the base (to
emitter which of course is grounded as is the other reluctor terminal).
Put a collector load resistor to the supply (5V) and you should have a
rising waveform when the reluctor output goes negative.

  Cheers,
        Paul B.