How to monitor data exchange on single wire?
You should be able to use a USB-based logic analyzer just fine. They are designed to 'snoop' in on signals without influencing them. 

If you look at the specifications of, for example, the Saleae Logic, you can see the input impedance is noted as: 1Mohm || 10pF 

This is a fairly high impedance, and that's good. A low input impedance would load down the signal you are trying to examine. The resistive component of the impedance will not change with frequency, but the capacitive component is frequency dependent. It probably doesn't matter in your case, but it's something to keep in mind. As the frequency of the input signal goes up, the logic analyzer will load the signal more and more.   

Another way to describe this behavior is provided here, which comes from an Agilent app note. 


  The logic analyzer probe has a high input impedance. The probe-tip
  circuitry consists of a tip resistor on the order of 20 kΩ. At low
  frequencies, the probe impedance will look like this resistance. As
  the frequency rises, parasitic capacitance in the probe will start to
  lower its impedance. The impedance will roll off following a standard
  RC response. This could present problems for the target system; as the
  probe impedance begins to approach the system impedance, the voltage
  divider formed with the probe becomes substantial. A low impedance
  will absorb the majority of the signal and cause system failure.