How to measure output DC impedance of clock oscillator?
I would load it with various resistors in series with a cap to GND and measure output voltage variations in the flat part of the waveform.

Responding to your edit:

If the spec is concerned with output impedance, then it is all about transmission lines and reflections, and what matters is the output impedance of the buffer on both signal levels. You can use the method I suggested to measure it.