Tachometer IC that does not require zero-crossings?
I realize this circuit requires a VR pickup sensor, and so the
LM2907/LM2917 must rely on triggering off of zero-crossings.

The data sheet tells you that a zero cross does not trigger anything: -

You need to exceed a positive threshold and then return back below a negative threshold or it won't register anything - this is to avoid noise triggering causing a false count on the tacho circuit.

But what if I have a square, 0-5V TTL input waveform?

AC couple to the input with an RC network is the usual ploy and note that the 8 pin version can have a negative input as low as -28 volts with respect to ground: -


Upon further reading of the datasheet, it talks about triggering off
of a differential input using the 14-pin version of the device, and
further explains this in section 10.1 paragraph 2 (page 13).

If you want to put a 0 volt to 5 volt square wave in then set the &quot;freed-up&quot; and spare input to a DC voltage of 2.5 volts or use something like this: -

Basically the circuit above is called a data slicer and converts the average signal into a DC signal for use on one of the comparator inputs.